That's tough. I would really have to think about which kind of game I wanted to make.
If I went with a strategy RPG like Valkyria Chronicles or Final Fantasy Tactics then it would definitely be Sony. I feel like they're supportive of a lot more genres than either of the other two. Actually, though, Nintendo looks like they're trying to change that, now that I think about it.
If it was more like Fallout 3, a western RPG, then probably Microsoft. If it's mainstream enough, I think they'd support it big-time. Still, I think I'd be afraid that they'd try to 'dude-bro' it up, which would suck. Thinking about that, I'm not sure I'd go with Microsoft.
If I went into it not knowing exactly what kind of game it would be, then Sony. I'm a big fan of Worldwide Studios and I love the variety they bring to the Playstation brand.