Personally, I think the whole XBLA and PSN thing should be stalled for at least another year. Till then, we can be pretty much sure that Aquaria has all its bugs worked out (imagine the time paradox bug in the XBLA version) and all "necessary" features in.
Also, by that time, PS3 will have gained MUCH in terms of popularity and PSN would be just as good a choice as XBLA.
It also should be on the system that most easily supports Mods.
WiiWare is probably the worst choice, due to the limited space and Nintendo's refusal of releasing something like a HD. I mean, sure, you can swap all your stuff via SD-card, but do you really wanna fiddle with File Management each time you wanna play Aquaria?

Also, I doubt that the Wii would allow Mods at all. I mean Nintendo is so overly protective over everything that you probably won't ever be able to put some mod-data on your Wii.