I don't have an ugi ball, so I can't directly speak on it. I never paid attention to them at all until the pre-sale for the firm's zip trainer was going on. Then I was interested in both, but ultimately decided on the zip trainer for it's ability to increase/decrease the weight as needed...and you can unzip it to use for step and other various things.
I've finally been able to start doing the workouts that came with the zip, and so far I am
it all! From what I have seen, the zip trainer stays more round vs. the ugi ball that kind of gets flattened out a bit over time? But I think overall you can use the zip trainer in pretty much almost all the same ways as the ugi, with the one exception of probably putting yours hands on it while doing pushups. But in that case you can use the zip opened up on the floor. One thing I wish the zip trainer came with was a book or chart of some sort with lots of different ways to use the zip trainer. The ugi ball comes with that. However, the zips workouts give you tons of various ways to use it. Plus for more ideas, you can always find ideas of how people have been using the ugi ball, or medicine balls and then see what works for you with the zip!
At any rate...in the end I decided to get the zip and so far I'm extremely glad I did. Also, I figured I have 60 days, I believe it is, to decide if I like it enough to keep it, and return it if not. So I didn't see it as a loss, and only a potential gain!