So, how does the tape emulator module in Sonar compare to the Waves J37?

Different beast.  J37 is the Abbey Road Tape Machine Model.
and has different color and saturation.  I like the sound of the J37.
Just all depends on "what" you're after...

As cclarry says, the're different.

The J37 offers a lot of options and great deal of control - input/output levels, various tape stock and tape speed emulations and a pretty decent basic tape echo function thrown in. It's become my "go to" tape emulator. J37 and the T12345 and Redd console emulations are a quick route into the "British studio" sound of the 1940s, 50s and 70s.

Cakewalk's is much simpler, but it's also much cheaper and uses nothing like as much cpu. Waves fancy animation of rotating tape spools looks good but once you've a few instances of the J37 running the cpu hit becomes very noticeable even with the visual gimmic turned off. And for tape plugins to have much effect they need it be on every channel plus at least the master bus, just like in a studio using a multi-track tape recorder.

Have a look at TB Reelbus for a far less-expensive ($21 vs. $300) alternative to J37. 

As with most things Waves J37 turns up on sale for a much more reasonable price pretty often. I forget what I paid for it but it was nothing anywhere near $300. I've never paid Waves full price for anything, come to that.

J37 is very sophisticated and can function as a tape delay. You need to decide what functionality you are after, as this will help narrow down the list of tape emulating plugins to just those you 'can't live without'. That said, I'm happy with Sonar's tape PC module and occasionally use Magnetic II, which is my favorite of all the tape emus.
FWIW, Acustica Nebula free has a very nice tape preset.

+1 for Magnetic II, I got that on sale a couple of years ago and use it all the time. It has a number of different tape emulations on it and sounds great. FWIW I don't think you strictly need to use tape emulation on every track for it to have an effect. Sometimes tape emulations are very effective on a select few tracks or even just one. You might just stick one on your drum bus for flavor, or perhaps you have a harsh sounding synth which needs smoothing out. 

My favorite J37 feature is the tape delay. Sounds really good.

Cheers,
Eddie
 

good advice... I missed the special that cclarry posted  a couple of weeks ago.  dang.  I will prob now get the TB Reelbus for now since that's fer cheap, but I like using my TG12345 and Redd console for many songs, so I thought it would be a good fit, but not for $200+.

Maarkr - Since you have a couple already, check with Koby at EveryPlugin.  He can quote you an upgrade price for the Abbey Road bundle which I did very reasonably during one of the sales.  He's usually the cheapest.