If you have never worked with a 3D editing package and you have never created levels before, then neither one is easy. Making 3D maps takes some practice and experience. Your first few maps will probably suck, and then get better over time. I suggest trying both and see which one is more agreeable to you as an editing methodology. Note that on the Quake side of things, you've got multiple choices for editing software (BSP, Worldcraft, QuakeEd). I think (not sure) that for Unreal, you've only got UnrealEd.

I have never tried to create an Unreal level, but I do know one maddening thing about making Quake levels that will probably give you fits the first time you try: VIS LEAKS.

See, the Quake engine speeds rendering by dividing up its levels into something called Binary Space Partitions (BSPs). These are essentially database entries which say "If the player is standing here, then he can potentially see polygons here, here, and here, so render only those polygons."

It creates this BSP tree mathematically, using a series of postprocessing tools. The problem arises if you've created a level with holes somewhere in the architecture. For example, a complex arrangement of vertices in a ceiling corner might have a gap between two walls that you didn't know was there. On the other side of that gap is "mathematical nothingness", an area of the map which can't be properly divided into the BSP tree. The result of such a "leak" is that the entire map isn't VIS'ed and every polygon is always fully drawn for every frame. So you get dismal frame rates.

Some poor level designers resort to encasing their level inside a giant sealed "box" to get rid of the VIS leaks. This solves nothing, since it doesn't fix the leak, and results in a properly VISed level that still has dismal frame rates.

Not having created an Unreal level, I don't know how this is handled in their engine. Anyone have experience with it?