The main difference is that American Roulette has a second Zero, giving the roulette wheel 38 pockets instead of 37. This double zero field increases the house edge from 2.7% to 5.26%, which gives the house a bit more of an advantage.
Other than the double zero, American Roulette is faster and players have less time to place their bets. Additionally, the betting layout in French Roulette is written in French. If you want the same rules as French roulette, but minus the language, then check out European roulette.