I have the Super Everdrive. I've considered the SD2SNES but couldn't justify the cost
I'm sort of in the same boat. Also I just picked up a copy of Ballz 3D so I can at least have a DSP1 chip ready if I end up buying one.
The way I see it, AS OF RIGHT NOW, the SD2SNES only has access to 8 games that the Super Everdrive doesn't have assuming that you install the DSP1 chip on the Super Everdrive. HOWEVER, that might change in the future. They are working on Super FX which would add another 8 games and they are planning on S-DD1 and SPC7110 which would add 5 more games. This of course doesn't include any repros or non-licensed games.
But if you buy the SD2SNES, you have to assume that the additional games might not ever happen. So I'm sort of torn, but leaning towards Super Everdrive.