Hey mate I'm somewhat of a synth geek so here is what I think
I've got an UltraNova and I can tell that I've heard fatter sounding oscillators on iPhone apps, so if you buy one don't be hoping for a sick dubstep bass machine (what it was marketed as). However what it is; a digital sounding beautiful soft and elegant pad synth which can also come out with some pretty awesome techy basses.
The Gaia, I have played once or twice and was kinda impressed with it but not blow away, and it's architecture is pretty backwards- it's made like 3 simple one oscillator synths and I've heard a few people complain it's a hindrance.
Other synths I recommend as opposed to the other two:
-Wardolf Blofel: it's such an awesome sounding synth (it's oscillators are so good and filters are too), is pretty easy to patch via a editing matrix, like the one on the microkorg along side that it has a beautifully sharp screen which helps you see whats going on and ontop of that it's 16 part multi timbrel.
-Arturia Minibrute and Roland SH 101, these are both awesome little analogue monosynths that sound perfect in every way!!
hope this helped you mate any questions and I'll answer them!