Subjectivity mode ON:
Skip the softsynths, you can download free demos and try them all out, unlike hardware. If you find one you really like, you can always get it cheap.
And skip buying new, you will get more synth for your money if you buy used, and if you wait for a good deal you won't get hit with the depreciation if you decide to resell later.
If you're OK with a monosynth, I'd take a Korg MS-10 over a Slim Phatty any day. (Subjectively, I'd also take an MS-20 over a Voyager.) But you said you're a guitarist and pianist, so you might be frustrated by a lack of polyphony.
If you're OK with digital, I'd recommend a used Virus or Nord Lead over the Gaia as well. Either of those will run circles around the Gaia. A used Virus Indigo looks like it could be found for less than $1000 used on eBay. A Virus kB also sold for less than $1000 and it has a five-octave keyboard. Again, since you play piano, you'll probably end up wanting a larger keyboard than the Gaia (or Virus Indigo) has so the Virus kB or Nord Lead 2X will give you more range.
If you're not OK with digital, I'd stay away from the DSI stuff. I had a Mopho and although it could make some cool sounds, I felt like every patch had the same tone color. If you want a new-ish analog poly, sadly, there just isn't that much available so finding something pre-1985 is your only choice. Any of the Roland Juno series can be had for well under $1000 and they're great to learn synthesis on. I would go for the Juno-60 personally (having owned one, and played a 106, I find the 106 lacking somehow). Just make sure you get something with tons of controls, because changing parameters as you play is part of the fun of playing a synth.