UDK doesn't allow you to create a commercial game though. I think they take 25% from your profit or something. Unity, on the other hand, allow you sell whatever you make. They only force you to buy pro version if you income from the product is more than $100,000 a year or something ( but who cares how much the pro cost since by that time you'll probably has more than enough to pay D: )
I've also read from the TIGs forum that UDK is generally more difficult to use and would be more suitable for the coder, while many artists/ designers seems to be able to handle Unity3D quite easily.
But of course, I haven't tried UDK so I can't say much. I've tried Unity though but it isn't that easy to start. I guess I have to give it more time, since this is also the case with Game Maker for me where when I first got it I have no idea how things work and what to do.