The Wii edition of RE4 is hardly a reason to own one, but there are enough upcoming games to warrant a purchase. The price is right too, and with its user base lead, it's a good system to have.
As for the other two, I'm not sure if I'd recommend getting both. Having owned both (until my 360 gave up), you'd be happy with one or the other if you primarily play 3rd party games. Look at the exclusives list for both and decide.
If it weren't for Tekken and MGS, I wouldn't have kept the PS3 especially with DMC4 and VF5 going multiplatform. Sigma is a good revision, but lacks the polish of Black. I'll pick up a 360 again when NG2 is out, but not anytime soon.
I'm not the big gamer I used to be, so it's down to a handful of franchises. Tekken, MGS, RE, DMC, and NG.