I've used both, and I came back to Omnifocus. I outlined my thoughts in a blog post at the time
http://www.safeism.com/blog/index.ph...cus-vs-things/
In short I didn't like the tagging feature of Things, and I found the lack of flexibility to hamper my ability to manage a whole-life to do list. It is simple and attractive but just not powerful enough for me.