Order of priority versus justifiable expenses versus disposable income. That would be the only real answer for me, or I'd have everything.
As far as my order or priority goes:
1 - PS4: I'll get one this year. I like Sony first-party and I like diversity.
2 - 3DS: I want one because of Fire Emblem, mostly, and a few other RPGs. I'm not a fan of the actual hardware, though.
3 - Xbox One: Honestly, at this point, Halo is pretty much the only draw. Quantum Break has a chance at grabbing my attention, however. Other than that, though, I would rank it far behind the PS4, as it is very unlikely to get much Japanese support. I'm not a Gears of War fan.
4 - Wii U: Zelda ... and that's probably it. Maybe XenoX but I haven't liked what I've seen of the combat system. A big maybe on Splatoon and Star Fox. If those end up being really good, it could leap-frog the XO. Generally speaking, though, I'm not a fan of platformers, which makes the Wii U far less attractive to me than to someone who is a fan of that genre.