Heres my take on this, some Evo 3D users may not like this.
Evo 3D
+ 3D Screen
+ Higher resoltuion Screen (540 x 960)
Galaxy S II
+ SAMOLED Plus
+ Faster CPU
+ Faster GPU
+ Better camera
+ Higher quality video recording (1080p 30fps)
+ Thinner and lighter
+ Storage (16GB vs 1GB)
+ Better stock video/music players, supports more formats
+ Better battery life (even though it's a smaller battery)
+ Very durable (You would be supprised how much punishment this thing can take)
There are also differences which don't fit into either, like Sense vs touchwiz and the choice of materials used.
Personally I'm not bothered about material, so long as the phone remains lightweight and durable I'm happy.
Touchwiz is incredibility smooth especially with the ICS rom where as sense is a bit of a resource hog, HTC even admitted as much.
That said is it a good swap? Depends, if it was just a straight swap than great, if you had to pay than it depends on how much you spent.