Be forewarned: I've been anti-contract guy ever since I bought the Nexus 4 and went prepaid.
Between those two options, I would advise in buying the Nexus 4 right now. Being off contract not only means freedom to move about carriers and MVNOs and whatnot, but it also keeps you free of being hassled by the carriers with fees and stuff. The power you give to the cellphone companies when you sign a contract is immense. They can basically do whatever they want and you have to deal with it. If you're not under contract, they won't pull stunts like assess a random $30 fee because you didn't cancel a trial period of some service they automatically and quietly signed you up for. I'm just all-around done with being, how should I put this, taken advantage of by the big cellphone companies. And especially with a cell-phone with hardware like the Nexus 4's for a price of close to what you'd pay for the One on contract, I think that should clinch it. And that doesn't even take into consideration the development community around the Nexus 4 (I've not been over to the One's boards so I don't know how big it is; I just can't imagine it'd be bigger and more engaged than the Nexus 4's. I could be completely wrong though).
I think the real question you should be asking yourself is this: should you buy the Nexus 4 now, or keep your One X until the next Nexus phone (supposedly) comes out in a few months.