Do you primarily use hand tools or power tools? Also, are you looking for a primary work bench, or an assembly bench?

Hand tool benches, you want them to be really heavy and sturdy since they get loaded in shear a lot by things like planing and sawing. For a hand tool bench, you basically need to decide what you have to work with, and what your work style is like. I like go be able to just clamp stuff to my bench top, so a Nicholson bench is a little annoying for me. Also, think about the space you have available, and whether you are right or left handed. For handtool work, I would prefer a face vice and a tail vice, with plenty of dog holes.

For power tools, the name of the game is modularity and mobility. Everything should be the same height and on wheels so you can move stuff around to act as infeed/outfeed tables. They don't need to be as heavy or sturdy, so you can use some space under the bench for more efficient storage. It's also nice to have a few ways to clamp other tools down.