are they even in the same ballpark, understanding the controls are quite a bit different?  Which one would you choose, if only were to use one?
 
And to throw a curve ball, does something low cost like the hardware ART Pro VLAII compressor (for a about $250 new) even compare?

The two soft comps are based on different topologies - the 76 modeled on the FET 1176 which is a "fast" compressor in that it has a quick attack, if you want.  The CA2a is modeled on the LA2A optical compressor, which used a light source to more gently ramp up the attack - more of a slope than vertical line.  Both are useful.  The 1176 is generally used on snare, kick, bass, acoustic guitar, vocals.  Anything you want to add some smack to.  The LA2a is usually used on vocals and bass and a lot of acoustic instruments that need a more natural sound rather than the Fetish on/off.  An old trick is to use the 1176 to grab the lead vocal and keep it steady, while shaping it (envelopes) with the LA2a.  The PC 76 is a good soft comp and comes w/ SONAR Pro.  The CA2a is a paid effect.  I like the optical better, I guess, but if the choice was between Pro and the optical, I'd go w/ SONAR Pro not because of Fet comp but everything else that comes w/ Pro.
 
I've heard the Art but not really used it.  It seems OK, and some people like it better w/ other tubes.  The best buy in comps for me these days is the new Warm Audio WA76.  It is in a different class price wise, but also sound.  Transformer coupled, high-quality electronics which makes stuff sound good just going through it.  And you can get the sound you've heard on records (and CDs) running your instrument through it.  I mean, exactly that sound and you then know how they got it.  It is quite a turn on for $600.  And that was going into the recording interface, which is the best thing about analog.  You can compress it going.  I don't know how good the art sounds, but the ability to pre-shape your instrument going in means you don't have to hit it so hard in the box, which can give you a more natural, relaxed sound during mixing - analogous to the "chained 1176/LA2a" trick on vocals, where neither unit has to work so hard. 
 
@

Well said.

Comparing a VST plug to a hardware unit like the Art Pro II is like comparing horses to cows.  On saying that if you want a low-cost outboard compressor, then the Art Pro is a fantastic piece of kit.  It can be used on the input for tracking, it can also be used as a mastering compressor. Be carful of the lows and low-mids it seems to add a little mud in these areas.
 
Look it's about the operator not the tools these days, I suspect a knowledgeable person can make the Art Pro sing but the less knowledgeable person would complain and say it is crap.
 
Ben  

I have an ART MPA II mic pre, with better tubes it does quite well for me. I also have the Warm Audio unit on order and should have it by May 1st. A friend in the industry demo'd one and said it was fantastic for the price.
 
As for the Pro Channel stuff. IMHO the PCa-2a is one of the best emulations of the real thing that Cakewalk has, AT summed it up well with his description on how to use it. The PC 76 also holds its own quite well when compared to the Bomb Factory & Soft Tube versions. 
 
There are videos on how to use both together as AT described on youtube, just not for the SONAR modules.
 
 

I had the original Pro VLA many years ago.
One quirk it had was on high-transient material, it would miss the first peak (it would shoot thru).
That was annoying at best...   
The Pro VLA is more akin to the LA-2A than the 1176... as it's meant to provide softer/gentler control of dynamics rather than clamping down (like the 1176).
FWIW, I don't miss the Pro VLA in any way.  The better software based LA-2A style plugins are more consistent (no peaks shooting thru)... and sound every bit as good (if not better).
 
Since the LA-2A and 1176 are radically different, one really isn't a reasonable substitute for the other.
I'd base your choice on which you need most (aggressive or soft).
Ultimately, it's a good idea to have both... as the combination covers most dynamics processing needs.

Dan Gonzalez has a mixing with CA2A mini video series posted on the cakewalk blog if you want to check it out.

thx guys.  great replies.   off to check the vid.

Yes Dan does a great job on this series of videos. But it covers the CA-2A and doesn't do a comparison of the two mentioned compressors in this string. 
 
Dan, great job with this video series; please keep it up, I know how to use the '2A but could always learn more about this great compressor!
 
I've got them both and as it has already been mentioned... you really need them both for different reasons and needs.
 
I  use the CA-2A for vocals and really like what it does to help my vocals.

FWIW I almost always use the CA2A for my vocals and the 76 for bass. For other sound sources, it depends on the intended result.