PVRTC sucks, period, not just for transparency. It's somewhat passable for texturing 3D objects on a low-res device, but that's it. Its artifacts are extremely noticeable, and would just kill the look of your UI. For UI I would suggest just sticking with the truecolor option, and finding ways of reducing the size of your sprites instead. Using sliced sprites and making your UI elements modular (made up of several different smaller components) can greatly reduce the size of your atlas.