I also have a UCD with a Magnum .91xls. I have used an APC 14x4W, 15x4W and APC 14x6. I settled on the 15x4W because I like larger props with lots of prop-wash, spool up is a little slow but still adequate. Both the 14x4W and 15x4W gave excellent pull from a hover.

I still want to try an APC 15x6 but haven't bought one yet.