Try Rivals mode. I have made more money and possibly more experience there than through the career / event list. You're limited on your car selection in career mode.
Online events and arcade mode (or free race, not sure what it's called) allow you to race all cars that are available in Forza that are within the regulations for that event.
The only plus for me for leveling up was getting the bonus cars as some are very expensive, then they can be upgraded or tuned. After level 50, it's just points to put in a pile with credit rewards for each level you obtain.
If anyone is ever on and wants to run some races, just let me know.