PopStarWannabe

TranceCanada
... compared to Z3TA+2 the sound quality of the original Z3TA+ is really not great

That was one line they marketed Z3TA 2 with, when it came out. It is absolutely not true!
 
I opened presets from the z3ta Classic installation sound bank in both versions. They sound absolutely the same.
 
I also designed couple of new presets from scratch in both synths using the same "recipe" and again, same sound.

Interesting, I did do a side by side comparison when I first got Z3TA+2 (and it's been quite a while since then) and for whatever reason at the time they did sound different to me, the original Z3TA+ sounded kind of muffled to me compared to Z3TA+2, however going back to try another comparison after your message I don't hear that difference I thought I remembered hearing the first time. 
 
Although I do rely heavily on presets and I have found enough of a benefit with the added availability of expansions in Z3TA+2, mostly cause when Z3TA+2 first came out a few of them were thrown in as a bonus

TranceCanada
 
Interesting, I did do a side by side comparison when I first got Z3TA+2 (and it's been quite a while since then) and for whatever reason at the time they did sound different to me, the original Z3TA+ sounded kind of muffled to me compared to Z3TA+2, however going back to try another comparison after your message I don't hear that difference I thought I remembered hearing the first time. 
 

 
Could this be due to the presense (or absence) of internal oversampling? It makes a pretty big difference with some patches.
 
As far as I can tell, their features are more or less the same. With Z3ta+2 you get a nicer interface. All you lose is the audio input, if you wanted to use Z3ta's FX on other audio. But I haven't heard anything there I couldn't replicate with other plugs. If you could use input as an oscillator/mod source, then it'd be something!