OK sorry, I was under the assumption that you did not have the revenue to carry both full time. Not that you can only afford one and not have enough left to contract someone on a short term basis. That is a tough one.
Now this is coming from my context and it has a lot to do with how we develop applications, but we do not do server side UI development for the web anymore, we do all of our UI work with JavaScript that communicates to REST services on the back end (we build web apps, not web sites).
Given that disclaimer, I personally would go with the UX designer, so long as s/he is a UX developer that writes the UI. An average back end developer can usually stand up some services that will suffice.
For my group, our reputation is that we can provide web apps that simplify business workflow or increase customer conversion so it is our bread and butter. Please keep that in mind because it naturally creates a bias in my logic.
As well though I think a UX developer (that only does UX) is less utilitarian and therefore if you can afford it, it would be the highest candidate to hire a short term consultant to provide the brain dump for someone else to implement.