Verizon > Sprint > AT&T

From the reports I've read and people I've spoken with, the 4G performance on Sprint isn't as pronounced as you would expect in terms of speed, but it is solid in terms of connectivity if you are in an area that has 4G. But if you are buying the phone with 4G as the primary reason, you'l probably be disappointed for awhile yet - and by the time 4G is really ready for a majority of users, you'll be looking at your EVO as 2 or 3 Android evolutions old.

But if you are just really interested in the EVO as a device itself, and accept that you may/probably won't have 4G most of the time unless you live within the initial coverage area, the EVO is a kick-ass phone.

But all things being equal, the Droid Incredible on Verizon is THE combination to have right now. (Unless you really don't like the HTC interface overlay and like a "pure" Android implementation - in which the Droid is still the best available in that regard as of today.)