I have a few opinions on this:
Opera MOBILE is more functional.
Opera MINI is much lighter in resources. A phone with low RAM (like my Treo 750) is more suited for mini, but mobile still works well.
I like the UI of Opera MINI better, most of mobile's options are hidden in menus that I haven't even wandered yet.
If mobile can find a way to be a little smoother (faster zoom time, smoother scrolling) without sacrificing more RAM, it will be awesome. Still doesn't come close to Safari, but add flash and we might have an argument for that
