I have the same problem, I have a Wii but will buy a second console (like I had a PS2 next to my GC). I bought the PS2 for the RPG's (PS1 and 2) the rest I played on my gamecube.
I think of the coming holiday Wii has by far the best line-up (Zack & Wiki, SMG, SSBB, Manhunt2, Ghost Squad and my favorite Fire Emblem) and there are RPG's and andventures enough in the future, so my second console this time has to be for the "hardcore" games like Splinter Cell, Assasins Creed, racers (GT5 or Forza) football games (FIFA or PES) (yes it's called football, you play it with your feet, while American Football is played mostly with your hands so it should be called something like Rugby for sissies)
It looks like the 360 has the best line-up now with Bioshock, Blue Dragon, Fable 2 (1 was cool but too short, finished it under 12 hours), Lost Odyssee and Splinter Cell exclusive and PS3 remains to be seen. But FFXIII might be great again (although I hated XII) and Kingdom Hearts migh come to PS3.
So a tough decision, does anyone know whether 360 has more RPGs and adventures coming? Or does PS3 will upgrade the level of its games, also the multiplats which look better on 360 right now.
Maybe I'll better wait another year and enjoy my Wii. Which fanboy can change my mind?