Like was said earlier I would steer you toward a profile. A Fusion from OMP will out perform the other two mentioned and is easier to handle IMHO. I have not flow a ST50 but have flown a ST90. I am not a huge fan of it. It is a very stable flier and the tracking and knife edge are great. It is just not great at 3D. The UCD also has it's pluses and minuses. A lot of which depends on your style of flying. I like the fusion so much because it is easy to make you look good. It's not as pretty being a profile. But I think it would be a good next step for you. It will aslo get you ready for a Edge or Yak or Cap or whatever you go to from here.

David