As a long time FlashFXP user I can say UltraFXP clearly build on the populatity of FlashFXP.
The GUI is very very similar. That includes layout of windows and menus.
They clearly just recoded FlashFXP.
FlashFXP is very stable and 99.99% bugfree.
Something that can't be said about UltraFXP.
The only reason to consider using UltraFXP is it's tabbed interface.
All IMO ofcourse