I am a heavy user of Omnifocus and love it. I did try the trial of Things, but it just didn't meet my needs. For example, Things doesn't support "start dates" for tasks, and Omnifocus does. That's huge for me--I use start dates all the time and find that it's instrumental in getting everything out of your head that is crucial to a GTD implementation. As an example--say my mom's birthday is 2 months from now and I want to put in a task to mail her bday card. I'll get that task out of my head by entering into OF, but I don't need to see this task all day every day for the next 2 months, so I'll set a start date around 6 weeks in the future, so when the time comes, the task becomes active and I can get it done. Similarly, if I need a report from Bob, but Bob is on vacation for 2 weeks, I can still record the task with a start date 2 weeks ahead, and I don't have to look at a task that I can't do until I can actually do it!
In terms of overall functionality, OF beats Things hands down, but not everyone needs OF. Things does have a nicer UI (OF2 will be released this year with a much improved interface) and may offer enough functionality for you. You should download both trials and check them out.