Just a quick note on this as I've owned 2 supernova 2's over the last year. I personally love the sound of the SN2, BUT (and it's a big but) if you're going to be using it for fast basslines, drums etc, you may find that the timing gets sloppy fast. This becomes even more of a problem when editing sounds whilst your sequence is playing (almost as if the effort involved in using the control surface is over stressing the CPU). *Jexus refers to this in his review*.
I initally thought I had a defective unit so bought another - still the same. I then thought it may be my software so tried it with various hardware sequencers and eventually the MIDI God that is the Atari ST!!! Still had poor timing.
The good news is that its little brother the Nova doesn't suffer this and it about half the price (and still has 6 outputs and 6 part multi). Sound is a bit more raw than the SN2 (slight aliasing in the higher notes) but once you get the filter going, you really don't notice. Side by side, I felt that the SN2 sounded slightly 'repressed' vs the Nova.
I liked my Ion and the timing was nice and tight. Useless effects though and it didn't really seem to excell at anything. Not really great for bass and lacking top end clarity for leads. You may want to get a Micron and and editor first to see if you like it and save yourself a few quid.
I only ever owned a Virus B and wasn't keen on the sound at all. Also found editing a chore, which was strange considering the amount of knobage! I know I'm in the minority there though - lot's love their Virus', it just didn't do it for me.
Sound-wise, I preferred the MS2000 over the Radias. I sold them both in the end though. Would take the Ion over either of them any day.
Have you considered the Blofeld? It's the only synth I've heard for a long time that literally made my jaw drop and made me think 'How did they make THAT sound'?!'. If you can stand the buggy OS, it's a killer machine and the interface is actually very quick to get around. There's also a free PC editor which is very ugly but very easy to use.