The USD75 Umik-1 and REW, is all I need so far. I've never even consider the much more expensive Omni mic until now that I am building my first pair of speakers and feel the Omni/DATS package could be useful. Before you pay premium for the Omnimic you may want to ask
@RichB who iirc, has experience with both. Are you interested in DIY speakers though, if not, is there something you feel REW cannot do for you?