it is worth considering that the licensing terms/costs are drastically different:
Unity has a free indie license, but the feature set is significantly crippled - my impression is that most serious developers will be looking at a commercial license instead. A commercial unity license costs $1,500 per developer (plus $400 for each of iOS and Android publishing, if you need those), but you never have to pay any royalties.
The UDK requires a $100 publication license when you are preparing to ship your product, but also charges 25% royalties on any revenue after the first $50,000.
This presents you with an interesting cost equation. Unity's per-seat licenses are expensive upfront, but after that you never need to pay a dime. The UDK costs pretty much nothing upfront, but if your game produces a high revenue, you will be paying a fair bit of money out in royalties. My back-of-the-envelope calculation suggests that the UDK might be more cost effective for a game that grosses under ~$120,000, and Unity is significantly more cost effective for a game that exceeds that figure - but you should run your own projections to validate the choice for your particular situation.
One other hard fact is that you will have to shed a huge amount of cash out if you need to change something in the source codes as well. I guess options like Ogre and Torque could be worth considering as well. In fact this link could help out also: free engines