It's easy really;
Aspects where FM beats GT:
- Career mode and race structure
- Cars on offer (quality, diversity and how appealing they are for the average gearhead)
- Livery editor
- AI
- Engine sounds
- Online selling of your cars to other users (auction house)
- Menu's (UI)
- Custom parts and painting
- Online
Aspects where GT beats FM
- Physics model
- Amount of cars on offer
- Amount of tracks on offer
- Day/ night time changes
- Weather changes
- Menu music
End score FM wins 9/6!
Both have their charmes, and even tough i own FM4 and it is definitely the better "racing game" i will buy GT6 (new or second hand), just for the pleasure of hotlapping on all those circuits with the great new physics model and day/ night cycles (yes that makes me feel warm inside).
And aside from all the fanboy-ism, i'm already looking forward to comparing their next installments in a couple of years time, because it will only get better from here + Project cars will challenge the both of them also 👍