I like radio shack but their trade in deals are pretty poor. Sprint buy back at for evo 4g (my phone) at evo 3d was $150, and I just checked, its currently $125 (pretty good for 3 months later). If you sign it now you have 45 days to return your evo 4g so its basically "locked in" @ $125 buy back by the time you got the SGS2 and then some. You could try ebaying your phone depending on what you have, and ebay is almost always a better route financially, but YMMV and its a hassle depending on your buyer, of course.
If I go with the SGS 2 (99% sure) i'll prolly just go the easy route and sell my evo back to sprint. $215 (w/tax) - $125 = $90 for 2.5x faster is a pretty good deal. Just a new contract