If you need to time-stretch audio, choose Professional (which offers AudioSnap and slip-stretching). The quality of time-stretched audio in Artist is very very low (if not improved from X3 base version).  

ProChannel is not part of Artist. If you had a previous version of Sonar that had ProChannel it WONT show up in Artist.

As someone said earlier, the framework for it isn't included in Artist. If I was working with some guys, say in a band then I reckon it would be worth the $99 to be able to freely swap files and collaborate.

Something to consider besides PC is that Platinum will have a lot more functionality as the year progresses. Right now it has Vocal sync. Later it will get drum replacer as well as other abilities that will not be in the other versions. The incentive is high to opt for Platinum.  

You probably want professional, the middling version.  It has Prochannel with the great sounding EQ.  And you'll find the PC is great for workflow.  Cake tends to put the PC stuff on sale, and the SSL buss comp and CA2A optical comp are worth having.
 
Since you already have Nomad effects and the Platinum synths you are paying for a lot of stuff you already have.  Platinum has some other things you'll probably find helpful and if SONAR becomes your main DAW you'll want.  The extra comps make the PC a one-stop shop. If you can upgrade cheaply it is worth it.
 
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Thanks all for the advice and opinions.  I went for the Pro version in the end.   This isn't my main DAW at the moment so couldn't justify the Platinum version.   I have a LiquidMix (bit dated) which provides most of my compressor/eq needs besides a number of other plugins covering the same character FX ground.

The point about the improved Audio Snap was important - I mostly do cinematic scoring work using samples however the collaboration this time is with a rock/acoustic players (a real band!) and being able to quantize audio without moving it into my main DAW will be important.
 
And now having had the chance to play I have to say that the PC EQ is definitely a pleasure to work with - I just wish the fly-out panel was part of Professional.  Still good value all round I reckon.
 
Cheers,