I'm a Macbook, MobileMe and iPhone user and I'm going to go with Things.

I tried both Omnifocus and Things. Omnifocus clearly has more features and settings, but I switched to Things because I wanted a simpler user interface. I was concerned, however, that it might be missing features.

While using Things, I have found that whenever a feature appears to be missing, it is because I'm trying to solve a GTD issue using features, whereas what I really need to do is re-read about GTD. I think the Things makers have a deep understanding of GTD because this has happened several times now. Omnifocus has the tools to provide you with many views of your information, but I found that Things had the views that I needed once I understood the method.

The good news is that both software do trials, so I'd do that if I were you on both.