RR3 may be more realistic, but it seems overall the last few years, most people on these boards prefer arcade-type racers. (Notice, I said "overall." I'm sure many would consider RR3 itself better than a bunch of the arcade racers showing up, even if they weren't into the other two in the series - but more people seem into the Asphalt and Need for Speed - and similar - games.) I love both of these games - but hate the limits on RR3. My only problem with A8 is the frustration of trying to get multiplayer going sometimes. Gives good money when it works, but sometimes you can't get a server connection OR wait forever for people to play with.
RR3 has better graphics, but jerky on the iPad 3 - and all of the fun is sapped out of it - which is a shame as I loved RR2. Asphalt 8 is much more fun, but really NFS:MW is the best of all. I voted A8 given the choices.