It depends. Do you spend a lot? I'd go with Starwood. But it doesn't sound like you do if you don't have a MC.

Do you fly a lot? Who with?

If you're not planning to use it, then I'd just go with the one that gives you the best deal for opening the card. Which, as far as I know, if United's 25K deal, which you can redeem for a domestic ticket. First year free, I think. Or just get a regular one with no annual fee.