Very tough between the two to be honest. Both are far from perfect but both are enjoyable to play, the throttle bug is annoying but manageable on a controller on Generations. Technically 10 is more solid than Generations which, aside from the throttle bug, has more than it's share of issues, none of which will get fixed.
I'll have to contradict
@Pfei though and say I found the tarmac to be much better feeling on Generations. On 10 I never felt like I could actually push the cars, it was way too unpredictable and twitchy for me and made me resent having to do tarmac events (to the point I would just skip the tarmac events in career mode). In Generations it felt so much better for me that the tarmac actually became enjoyable and Germany became one of my favourite rallies in the game.
The content is pretty similar between the two games, I can't remember exactly which cars are in Generations and not 10 (aside from obviously the hybrid rally1 cars), the 206 may have been one. Plus Corsica and a couple other locations that got bought in from previous games. 10 does have the anniversary mode which gives the historic cars something to do and was fun to play through. Career mode is completely identical but Generations does have a leagues mode for online racing which was very enjoyable, 10 doesn't have this and probably wouldn't have the player base to sustain it anyway, Generations is barely holding on as it is now.
TL;DR, both are good, both have issues, both have lots of content, Generations probably edges it for the content, multiplayer and (in my opinion) better tarmac driving.
