Firstly do you want a keyboard or a module?
Novation will be bringing out 3 keyboard versions of the Supernova and Nova synthes respectively but they will be expensive!!!!
I own an Z1 with 18 voice expansion and love it to death! Along with my DW8000, it's the most induvidual sounding synthe in my collection of gear!!
The Novation and Korg are 2 very different beasts IMHO! In terms of analogue synthesis emulation, both are extremely powerful. However, they both have their own unique sound too. The Z1 has the more 'Japanese' sound (cleaner) whilst the Supernova sounds more 'American' (grittier) in my opinion. This does NOT mean that one sounds better than the other. Both will do things like oscillaotor sync, PWM etc etc.
However, if I had to say that one was more powerful as a synthe, I'd have to say the Z1- WHY- it has 13 different oscillator types which not only do great analogue simulations but acoustic simulations as well (awesome, EP's, brass, woodwind, string, guitar, bass, clav, organs). Really everything but acoustic piano and drums. (I do wish Korg added a 14th algorythm to simulate old beatboxes like what is available now on their OASYS PCI card). The Novation really is designed to mimic the old synthes of the past and can't touch Z1 for the acoustic simulations (not even EP's and organs).
FX wise, this is the Z1's weak point IMHO- whilst there's a nice selection it's the way they are implemented that's bad (only having 2 insert fx into reverb/delay only in series even in multi mode). However, bear in mind that the Z1 was released over 3 years ago and Korg's main aim of this synthe was as a performance instrument rather than as a 'do it all' workstation with different fx on each channel! The acoustic simulations are nothing short of 'fantastic!' especially the EP's, acoustic guitars and trumpets. I also love the vocal sounds.
To be brutally honest, the Novation kills the Z1 in other areas- I think it's easier to program (not as deep a synthe as the Z1) and it has more real time control (even though the Z1 has great real time control also). FX wise it's MUCH MUCH better with each program being able to retain origional fx settings in a multi-timbral enviroment. Polyphony wise, the Supernova beats the Z1 hands down (20 voice expandable to 44 voice).
Really you can no doubt see the plusses and minuses of each unit can't you?
IMHO go for the one that SOUNDS BEST TO YOU and not only on features. I love the sounds of both these units but to me they sound so different from each other. I'D LOVE TO ADD A SUPERNOVA TO MY COLLECTION!!!! AND USE IT WITH MY Z1!!!! But I'd also love to check out the new Virus KB which some have said is a more capable analogue modelling synthe than both the Supernova and Z1 (I've not tried one so I can't comment). Check that unit out also.
Yes there are alot of options but in the end here is my advice to finish off-
a). If you're after a nice keyboard with awesome sonic flexibility from analogue to acoustic and don't need a massive amount of polyphony or multi-timbral power, get the Z1.
b). If the acoustic simulations don't get you going and you're after alot more multi-timbral power, polyphony and outputs (and you have a capable controller) go for the Supernova. The decision is yours!!!! Cheers.