I think Things is probably the best for people who have a lot of stuff moving quickly. It's very easy to get stuff in, very easy to reschedule. It maps very easily to GTD, and is quite intuitive.

Omnifocus is better for people who want detailed control over projects, with dependencies, sequential and parallel projects, project outlines et cetera. I personally find it a bit ponderous, but it's very good.

I don't care for Remember the Milk. The iOS apps are nice enough, but I find the web site hard to use. For a web + iOS solution, I prefer Toodledoo. It's a good choice for people who want a lot of flexibility to try different set-ups, and it's inexpensive.