I don't know much about Z3ta+ vs. Rapture, but I know what the difference is between a true VA synth and Rapture. Rapture is sample-based (even if it "redraws" the harmonics of wavetables to eliminate aliasing), whereas a VA synth (and especially good, recent ones) are truly emulating analog hardware. VA synths will have pulse width modulation, oscillator sync, various analog-style imperfections in pitch, signal quality, etc., FM, and other features that Rapture won't have. Rapture has excellent analog-sounding filters, but the oscillators are a completely different story. You may be able to emulate a lot of analog-ish sounds in Rapture, but certain things you simply won't be able to do. On the other hand, Rapture can take any short wave file and "rebuild" it to make it have the rich character of analog synths, while VA synths are limited to pulse, saw, triangle, and sine.
Basically, you'll want both types of synths, not just one or the other.
Arturia's synths are excellent, and Pentagon is as well. Haven't tried GMedia's stuff much. Another thing to look into is CreamWare's new Klangboxes, which are hardware rack units that plug into your computer and are about as close to software as hardware can get (they're not too expensive, either). They have emulations of the MiniMoog (they call it MiniMax), Prophet-5 (Pro-12), and Odyssey (Prodyssey). As I have the MiniMax (the Scope DSP version), I've compared it with Arturia's MiniMoog, and CreamWare's version sounds, IMHO, much better and has more character. Unfortunately, it's not multi-timberal.
Just a thought....
Well, whatever you get, I hope you have a lot of fun with it!
Shayne