Well if you plan on swapping now I would suggest Epic 4g
...here is why:
Arrive - no theming..its a locked down platform
Evo 4G - probably has the best dev community out of the 3 but the GPU is horrible and will be completely blown out of the water by the new hardware accelerated apps.
Epic 4G - Has the best CPU out of the 3 and the best GPU, the GPU is actually on par with an Evo 3d and other dual core phones. It also has a kb, if you dont want a keyboard there is the Nexus S 4G which is a pure google phone...
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if your going to wait then its a hard call...so far Evo3d is the only one Confirmed..spec wise SGS 2 is better but who knows when sprint will get it..and I hear by end of the year Nvidia is promising Quad-Core phones :/