I strongly recommend:
A) First, download a trial copy of OF. Before you jump into using it, watch the ScreenCast videos of using OmniFocus (http://www.omnigroup.com/applications/omnifocus/
). They are a great way to get a practical introduction in a short amount of time. Don't just "watch" the videos, follow along with your trial copy & try entering a few tasks the way he does in the video. It will really help you skip some of the learning curve most people struggle with when they try to just "jump in" to OF.
B) After you've watched at least the "Basics" screencast, take a small chunk of projects/tasks from Things & enter them into OF. Actually using the tool with your own tasks will give you the best experience of what it's like, much more than anybody else's advice. As you play with OF, try to avoid making it "work like Things". There are some significant differences in "how" these apps help you organize your stuff, and if you really want to figure out which is better for you, you should be sure to say "How does OF handle this?" instead of "Why doesn't OF do this the way Things does?!"
As for my own thoughts, I downloaded both OF & Things trials, and used them both for a couple weeks. They have a lot of similarities, and share many of the same benefits/features. You can do a Google search on "Things vs OmniFocus" to find dozens of different opinions about them. Largely, I'd say the choice really adds up to personal differences in the way people like to organize their work -- this makes the choice as much about each program's philosophy as its features. As for me....
I like Things' "tags" feature because it lets you assign multiple keywords/contexts to any given task, which is really useful, and something OF lacks. *However*, it also can let you get carried away with tagging, and I immediately found that tags seemed to clutter up the UI and distract me from my real goal: actually *doing* stuff. (you can hide the tags, but then I found instead of seeing too "much" information, I saw too little).
Unlike Things, OF imposes a little more structure on how you organize your work -- it follows the GTD principles much more closely than Things. People who want to organize work "their way" will usually prefer Things, for this reason. It just kind of lets you do what you want, without providing much structure. People who really found they like the GTD approach often like OF better, because it is structured around the GTD principles. Having read the GTD book, I really liked the structure it provided, and so I have found OF to be very natural to pick up & start using.
The *biggest* reason I chose OF over Things was the same one BevvyB mentioned: the iPhone syncing. Earlier this year, I started using MobileMe syncing for my contacts & calendars from my Mac to my iPhone (and back). It was so fluid & easy, I never had to think about it again. Currently, OF supports MobileMe syncing (along with a bunch of other sync methods), and Things doesn't. For someone who can practically no longer *survive* without his MobileMe calendar/contacts syncing, that was a big difference for me. Ultimately, this was the clincher.
Cost: a lot of people note that OF is more expensive to buy than Things. While this is true, having a system that really works for you and helps you be more productive is a very valuable thing, and at the risk of sounding like a salesman, a difference of $30 (or whatever it is) is really insignificant if you find the right tool for you. Compared to a Franklin Covey paper planner, these software tools are less than half the cost and are ten times as useful (not to mention a lot easier to carry around!).
Try out both tools & see which one feels right to you.