Well it is a good question.
I have personally bought every big todo app for the iPhone, Omnifocus, Things, RTM, Todo ect.
Todo I think has the worst UI of the bunch. I know a lot of people love it but I personally don´t get that, maybe because I don´t use Toodledo. It has some smart functions though like the way you can add new actions quickly.
Things is by far the prettiest of them all but IMO it is overly simple and CC has been waaaay too slow to implement basic functions like tags and areas. I know that they have just sent an update out that implements tags but it should have been out long ago. Still even with tags I think that it is too basic, no sub projects, no sub contexts, no use of location and no way to use Next Action.
RTM has almost all of the functions needed, except that you have to mark your next actions yourself which is a hazzle IMO. Apart from that it is very snappy and it syncs flawlessly. I have been using it since it came out. I would really like it if you could tweak your projects so that it would point out next actions itself.
Omnifocus is by far the most powerful with tons of options for tweaking so that it fits your needs. It automates next actions. It is a little slower to work with though, especially the sync process is really slow, and it can take quite a lot of time.
I am personally struggling with deciding if I should use Omnifocus or RTM as they both have pro and cons.