Feature-wise, there isn't as much in it as you might expect. The UDK has the edge when it comes to rendering, but I'd like to think that the gap is shrinking with recent Unity releases. Unity has a significantly easier learning curve, but the toolset isn't quite as mature. Unity has more publishing options...
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.