The multiplat owner's thoughts:
The PSN Store is by far the easiest to use of all the systems, with the Wii Shop channel and the Game Store about equal.
The PSN has the best quality-to-crap ratio, but not much quality overall. That said, the games that are quality are really great. WiiWare has the fewest quality offerings, but by far the most potential for original and unique software, and already supports genres the other 2 don't. XBLA has the most quality games of these 3, and in a variety of genres... but this is all besides the point:
This isn't even a contest for quality when you include Virtual Console. I'm sorry, Castle Crashers and Pixeljunk Monsters are great, but if you think they are a substitute for 2 decades of incredible back-catalog you are smoking something. If you don't include VC (although obviously you should), then you have a debate here, but when you do, the advantage is ridiculously overwhelmingly in favor of the Wii's downloadable service because of the sheer amount of amazing games.
I have already bought some 20 VC and WiiWare titles and I doubt in the entire time I own the PS3 and 360 I will buy half that on either service.
