Just curious what the differences are.

There is a spreadsheet somewhere on the difference between TH2 Sonar/Producer but I'm not sure if it has changed since X3. Basically with Producer you get some more presets and I THINK some extra cabs and maybe a couple other things. Producer is a little sparse but more that useable if you get creative with it.
 
The full version however is loaded with just about every sound for anything you could ever want to do. The Brunetti heads are awesome. I've never been a Randall guy but they are perfectly modeled. There are tons of pedals that cover anything I've ever encountered in the real world. Crazy rack effects and a buttload of presets. If you like how the freebie TH2 sounds and play a lot of guitar it's a good buy IMO.
 
I just hope they get the attention of some other brand names for licensed modelling but their "faux" models are still pretty accurate.

Thanks, Beep.

Here is some info about all three
TH2 SONAR http://blog.cakewalk.com/new-in-sonar-x2-essential-and-studio-scorching-guitar-tones-with-th2-sonar-from-overloud/
 
TH2 Producer http://blog.cakewalk.com/new-in-sonar-x2-producer-scorching-tones-and-killer-effects-with-th2-producer-from-overloud/
 
TH2 (Full Version) http://www.overloud.com/products/th2_model.php

Thanks for the links, Scook. Very helpful.

Have to say, I think I'm going to jump on an upgrade deal if Cakewalk offers one.  I have a number of amp simulators but am finding I keep gravitating to TH2.

I've always found the TH2 'sound' warmer than Guitar Rig, though I find GR more flexible. I prefer TH2 for actual guitars and GR for synths.
 
This is just my perception, I can't offer any science to back it up :)