The real race cars (single seater...) are much easier to drive than anything else.
If there is a car in this game that makes the most perfect trainer, its the MRT. I think its the most undervalued car in the game. Its loose on the gas, pushy on the brakes, has enough power to spin the rears coming off of a corner, its light enough that "overusing" the rumble strips will mess you up. It teaches you all of the basics, and once you master it, it will provide some of the closest racing you can get...
Aside from that... FWD is great. Its a great learning tool for really learning how to shift. Shift too early, and you push right through a corner (you can still shift alot earlier than a rear-wheel) but it allows you to learn to really work the gears without worrying about spinning it every time.
It's all about learning in a progress. If you're great at one car but suck with all of the others, you really aren't that good. I'd rather be competent and mildly competitive in all the cars than super competitive in one but none of the others.
I learned in the open wheel cars. MRT to FOX to FO8. FOX is still my favorite, mainly because its easier to get into a rhythm on most tracks...Im now learning the slower cars, because frankly, they are alot harder to drive. Its good fun though!