As as I see it, GT6, unpatched, is basically GT5 Spec II with:
+ 15% more cars and 20% more tracks
+ Improved driving physics
+ Marginally better graphics, sounds
+ Simplified interface
- Shorter and easier career mode, less credit availability
- Less features (course maker, B-spec, telemetry, 3D, etc, but also small things like lack of fuel/tire consumption on free run, no more tire load indicator, etc)
- Same usual PD inconsistencies (framerate, weater/time only on selected circuits, standard cars, limited amount of customization and not on all cars, etc)
PD are clearly placing their bets on online, DLC and updates, because as a whole GT6 is currently in a worse state than what GT5 was at launch, in my opinion.