The UltraNova is an excellent synth - there is no question about that. Editing on the UltraNova is not as straight forward as the Gaia clearly, as it is all done through menus, but neither is it a massive chore either. The sound quality is top drawer from the Nova, and in truth, possibly slightly better than the Gaia. One way that the Nova blasts the Gaia out of the water is in the software which ships with it. You get a plug-in and librarian out of the box (or free download from the Novation website) and the software plugin is absolutely excellent - one of the best I've seen to link software and hardware. Although I haven't been keeping up with current events, it seems that the Gaia Editor is still not fully available (and with a £90 price tag last time I looked is not really something I want to get into).
Having said all that - I am on a studio purge at the moment and am getting rid of some of my equipment. I have no hesitancy at all in getting rid of my UltraNova, but my Gaia is staying put. Really down to the simplicity of it, and the ease of creating / tweaking new sounds. Although the Nova is good, it just isn't (to me anyway) as inspiring as the Gaia - and that is what is important to me.