A few things:
1) Who has better coverage in your area? Where I am Sprint is fantastic, I never drop calls but I have friends on AT&T that I can never get a hold of because calls aren't coming through or they get dropped (or maybe they just dont want to talk to me
). However, when I was in a different part of the country in November (Ft Worth area) AT&T seemed to have better coverage than Sprint in some remote locations. Coverage is definitely a large consideration for me, and the cost of the monthly bill.
2) Get an EVO on Sprint's 30 day trial deal and do the same on AT&T if they offer something similar. Take em both for a test drive, then make your decision.