The main differences are basically down to one being a game that simulates FIA GT class cars (with rules on components/setups/etc) and one is for a road car. It is possible to port over cars that are similar-enough to each other without having the model - eg the MC12 and the Elise - and for the most part they are compatible if run with the same settings, though you have to watch out for a few model-specific settings that can interfere (undertray parameters). When you have the same car (or near enough) in each it looks a lot closer. Eg:
Here's Shift's MC12 aero settings:
[FRONTWING]
FWRange=(0.0, 1.0, 10) // front wing range
FWSetting=8 // front wing setting
FWMaxHeight=(0.10) // maximum height to take account of for downforce
FWDragParams=(0.05, 0.006, 0.0004) // base drag and 1st and 2nd order with setting
FWLiftParams=(-0.247, -0.0105, 0.00003) // base lift and 1st and 2nd order with setting
FWLiftHeight=(0.335) // effect of current height on lift coefficient
FWLiftSideways=(0.0) // dropoff in downforce with yaw (0.0 = none, 1.0 = max)
FWLeft=(-0.22, 0.0, 0.0) // aero forces from moving left
FWRight=(0.22, 0.0, 0.0) // aero forces from moving right
FWUp=(0.0, -0.50, -0.001) // aero forces from moving up
FWDown=(0.0, 0.25, 0.001) // aero forces from moving down
FWAft=(0.0, 0.02, -0.02) // aero forces from moving rearwards
FWFore=(0.0, 0.0, 0.0) // aero forces from moving forwards (recomputed from settings)
FWRot=(0.05, 0.025, 0.075) // aero torque from rotating
FWCenter=(0.00, 0.20, -0.60) // center of front wing forces (offset from center of front axle in ref plane)
[REARWING]
RWRange=(1.0, 1.0, 10) // rear wing range
RWSetting=8 // rear wing setting
RWDragParams=(0.05, 0.006, 0.0004) // base drag and 1st and 2nd order with setting
RWLiftParams=(-0.342, -0.0145, 0.00002) // base lift and 1st and 2nd order with setting
RWLiftSideways=(0.0) // dropoff in downforce with yaw (0.0 = none, 1.0 = max)
RWPeakYaw=(0.0, 0.00) // angle of peak, multiplier at peak
RWLeft=(-1.00, 0.0, 0.0) // aero forces from moving left
RWRight=(1.00, 0.0, 0.0) // aero forces from moving right
RWUp=(0.0, -0.70, -0.002) // aero forces from moving up
RWDown=(0.0, 0.45, 0.002) // aero forces from moving down
RWAft=(0.0, 0.03, -0.4) // aero forces from moving rearwards
RWFore=(0.0, 0.0, 0.0) // aero forces from moving forwards (recomputed from settings)
RWRot=(0.08, 0.05, 0.11) // aero torque from rotating
RWCenter=(0.00, 0.50, 0.85) // center of rear wing forces (offset from center of rear axle at ref plane)
[BODYAERO]
BodyDragBase=(0.30) // base drag
BodyDragHeightAvg=(0.22) // drag increase with average ride height
BodyDragHeightDiff=(0.90) // drag increase with front/rear ride height difference
BodyMaxHeight=(0.20) // maximum ride height that affects drag/lift
BodyLeft=(-0.7, 0.0, 0.0) // aero forces from moving left
BodyRight=(0.7, 0.0, 0.0) // aero forces from moving right
BodyUp=(0.0, -1.0, 0.0) // aero forces from moving up
BodyDown=(0.0, 0.6, 0.0) // aero forces from moving down
BodyAft=(0.0, 0.50, -0.8) // aero forces from moving rearwards
BodyFore=(0.0, -0.247, 0.435) // aero forces from moving forwards (lift value important!)
BodyRot=(5.73, 5.3, 3.00) // aero torque from rotating
BodyCenter=(0.0, 0.40, -1.50) // center of body aero forces (offset from center of rear axle at ref plane)
RadiatorRange=(0.0, 1.0, 4) // radiator range
RadiatorSetting=3 // radiator setting
RadiatorDrag=(0.005) // effect of radiator setting on drag
RadiatorLift=(0.003) // effect of radiator setting on lift
BrakeDuctRange=(0.0, 1.0, 5) // brake duct range
BrakeDuctSetting=1 // brake duct setting
BrakeDuctDrag=(0.006) // effect of brake duct setting on drag
BrakeDuctLift=(0.00825) // effect of brake duct setting on lift
[DIFFUSER]
DiffuserBase=(-0.900, -0.50, 5.0) // base lift and 1st/2nd order with rear ride height
DiffuserFrontHeight=(0.000) // 1st order with front ride height
DiffuserRake=(0.005, -0.0, 0.0) // Optimum rake (rear - front ride height), 1st order with current difference from opt, 2nd order
DiffuserLimits=(0.010, 0.100, 0.055) // Min ride height before stalling begins (0.0 to disable), max rear ride height for computations, max rake difference for computations
DiffuserStall=(0.0, 1.0) // function to compute stall ride height (0.0=minimum, 1.0=average), downforce lost when bottoming out (0.0=none,1.0=complete stall)
DiffuserSideways=(0.0) // dropoff with yaw (0.0 = none, 1.0 = max)
DiffuserCenter=(0.0, 0.00, -1.20) // center of diffuser forces (offset from center of rear axle at ref plane)
And here's the same car in GTR2:
[FRONTWING]
FWRange=(3.0, 1.0, 2) // front wing range
FWSetting=1 // front wing setting
FWMaxHeight=(0.10) // maximum height to take account of for downforce
FWDragParams=(0.0035, 0.00931, 0.0004) // base drag and 1st and 2nd order with setting
FWLiftParams=(-0.247, -0.1045, 0.00003) // base lift and 1st and 2nd order with setting
FWLiftHeight=(0.335) // effect of current height on lift coefficient
FWLiftSideways=(0.0) // dropoff in downforce with yaw (0.0 = none, 1.0 = max)
FWLeft=(-0.005, 0.0, 0.0) // aero forces from moving left
FWRight=(0.005, 0.0, 0.0) // aero forces from moving right
FWUp=(0.0, -0.05, -0.001) // aero forces from moving up
FWDown=(0.0, 0.025, 0.001) // aero forces from moving down
FWAft=(0.0, 0.02, -0.02) // aero forces from moving rearwards
FWFore=(0.0, 0.0, 0.0) // aero forces from moving forwards (recomputed from settings)
FWRot=(0.05, 0.025, 0.075) // aero torque from rotating
FWCenter=(0.00, 0.00, -0.80) // center of front wing forces (offset from center of front axle in ref plane)
[REARWING]
RWRange=(11.0, 1.0, 12) // rear wing range
RWSetting=9 // rear wing setting
RWDragParams=(0.0038, 0.00385, 0.0001) // base drag and 1st and 2nd order with setting
RWLiftParams=(-0.342, -0.0266, 0.00002) // base lift and 1st and 2nd order with setting
RWLiftSideways=(0.0) // dropoff in downforce with yaw (0.0 = none, 1.0 = max)
RWPeakYaw=(12.0, 0.90) // angle of peak, multiplier at peak
RWLeft=(-0.005, 0.0, 0.0) // aero forces from moving left
RWRight=(0.005, 0.0, 0.0) // aero forces from moving right
RWUp=(0.0, -0.09, -0.002) // aero forces from moving up
RWDown=(0.0, 0.045, 0.002) // aero forces from moving down
RWAft=(0.0, 0.03, -0.4) // aero forces from moving rearwards
RWFore=(0.0, 0.0, 0.0) // aero forces from moving forwards (recomputed from settings)
RWRot=(0.08, 0.05, 0.11) // aero torque from rotating
RWCenter=(0.00, 0.50, 0.85) // center of rear wing forces (offset from center of rear axle at ref plane)
[BODYAERO]
BodyDragBase=(0.27) // base drag
BodyDragHeightAvg=(0.22) // drag increase with average ride height
BodyDragHeightDiff=(0.90) // drag increase with front/rear ride height difference
BodyMaxHeight=(0.20) // maximum ride height that affects drag/lift
BodyLeft=(-0.5, 0.0, 0.0) // aero forces from moving left
BodyRight=(0.5, 0.0, 0.0) // aero forces from moving right
BodyUp=(0.0, -1.1, 0.0) // aero forces from moving up
BodyDown=(0.0, 1.1, 0.0) // aero forces from moving down
BodyAft=(0.0, 0.50, -0.8) // aero forces from moving rearwards
BodyFore=(0.0, -0.247, 0.435) // aero forces from moving forwards (lift value important!)
BodyRot=(9.73, 7.3, 4.87) // aero torque from rotating
BodyCenter=(0.0, 0.40, -1.50) // center of body aero forces (offset from center of rear axle at ref plane)
RadiatorRange=(0.0, 1.0, 4) // radiator range
RadiatorSetting=3 // radiator setting
RadiatorDrag=(0.002) // effect of radiator setting on drag
RadiatorLift=(0.0015) // effect of radiator setting on lift
BrakeDuctRange=(0.0, 1.0, 5) // brake duct range
BrakeDuctSetting=3 // brake duct setting
BrakeDuctDrag=(0.002) // effect of brake duct setting on drag
BrakeDuctLift=(0.00425) // effect of brake duct setting on lift
[DIFFUSER]
DiffuserBase=(-0.855, -5.825, 107.5) // base lift and 1st/2nd order with rear ride height
DiffuserFrontHeight=(0.000) // 1st order with front ride height
DiffuserRake=(0.000, -10.0, 300.0) // Optimum rake (rear - front ride height), 1st order with current difference from opt, 2nd order
DiffuserLimits=(0.010, 0.100, 0.065) // Min ride height before stalling begins (0.0 to disable), max rear ride height for computations, max rake difference for computations
DiffuserStall=(0.0, 1.0) // function to compute stall ride height (0.0=minimum, 1.0=average), downforce lost when bottoming out (0.0=none,1.0=complete stall)
DiffuserSideways=(0.3) // dropoff with yaw (0.0 = none, 1.0 = max)
DiffuserCenter=(0.0, 0.00, -1.40) // center of diffuser forces (offset from center of rear axle at ref plane)
(gah that took a while to sort out, I pasted the shift version over the gtr2 file locally here and spent ages thinking .. wait a sec.. I don't remember them being THAT close
)