Basically, they are offering to buy back your Evo for $125. It should be that amount regardless of the phone you choose to replace it.
Evo 4G is one of those rare Androids that was a huge leap from other phones being offered at the time. While the Evo 3D didn't have the same wow factor as the 4G, it's a damn impressive phone.
Screen is beautiful. If you get a chance, do a side-by-side comparison with your existing Evo and an E3D in the store. Huge difference.
Battery life is vastly improved compared to the Evo 4G. Though if you know what you're doing, it was possible to tame the 4G to go easy on the battery. I didn't have to do much at all with my Evo 3D, and i'm getting excellent battery life.
The 3D feature on the Evo 3D is entirely optional. It's there if you want it. Otherwise, you can use the phone sans 3D with no ill effects. But I encourage you to try it out. You might be pleasantly surprised.