Also depend on your driving style - if you are agressive 20K miles is pretty good.
If you are not "performance" driveing make sure you look for a little bit more of an average tire with longer life.
Kumbho Ecsta SPT are rated 320 tread life and General Exclaim UHP are rated 380
or the Goodyear Eagle F1 All Season would be a very good choice- rated 420. (VERY good choice if you drive in more than just summer - plus there is a $50 rebate running right now)
Good luck.