how different is the UDK from the engine itself?
Much better everything really. UDK I believe is just essentially a free cut down version of Unreal Engine 3. So now you just pay $20 a month and get access to Unreal 4, new updates, all the documentation, etc. Stop paying and you keep it but you just lose access to updates and other stuff.
Everything is pretty streamlined and easy to use in UE4 and pretty customizable. I know they change kizmat(correct spelling?) to blueprint which is much better. I would highly recommend just trying to get UE4 since it is super powerful. Just from the very little I have used of both, UE4 has been much easier for me to learn and navigate.