Everyone here is going to say BF2 is leaps better then Warrock because their nothing more then tools that business's like EA thrives on (zomg, I payed moneyz for BF2 so its def. better!). Best bet is to try both for yourself, or at least see if you like Warrock (since its free) and if you don't then just get BF2. Problem solved.
BF2 has things like the graphics over Warrock, for sure, but as far as gameplay is concerned it's really something to try for yourself.