Plenty of differences. For starters Open Topic is a hosted board which IP maintains. You don't need to upgrade it, but they do have different customization options. You won't be able to hack it, like you can with your UBB either. It does offer different features, and uses a database backend, instead of text files.
UBB as I'm sure you know, is customizable, and there are a number of excellent hacks which can add new features to your board. However, if you hack your board, you limit the amount of support you will receive from others, and upgrades are your own responsibility, as well as providing a server to run it on.
I wouldn't compare the products based on price alone. Consider what you want to do with your board, and how you want to run and maintain it...then make your decision.
-Tacks
