Fifa 10 has all the clubs (international and league) has more game modes (i think) and is only a few months older. From what I've heard Fifa world cup is a little easier and is aimed at the common fan.
Either get Fifa 10 or wait for Fifa 11 in my opinion
I liked this game alot.. It was my first FIFA game I bought but for the price of $60 just wait for the new one. After awhile you just have like nothing to do in the game besides qualify for the same teams.
I'm really craving FIFA 11 now though. I want to build a empire with New York Red Bulls.. Get Kaka, Messi, etc be the Man U of the MLS...