What unit you purchase is really a matter of personal preference. To help you decide we will give a brief overview.
There are really (3) different genres of 13 pin devices:
1 - Guitar Synth
If you want to play the sounds of other instruments like horns, strings, piano etc. and blend them with your piezo and regular magnetic pickups, units like the Roland GR-20 or Axon AX-100 would be best. The GR-33 is older has some great sounds but is less responsive than the newer GR-20.
The GR-20 is very easy to use. The Axon AX-100 is more complex to use but has faster tracking on the low notes.
These units actually do a Guitar to MIDI conversion then trigger their own built in sounds.
Roland GR-20 http://www.rolandus.com/products/productdetails.aspx?ObjectId=592&ParentId=53
Roland GR-33 http://www.rolandus.com/products/productdetails.aspx?ObjectId=237&ParentId=73
Axon AX-100 http://www.axon-technologies.net/modules.php?op=modload&name=News&file=article&sid=1&menu=101
2 - Guitar Modeling
If you want to emulate the sounds of other types guitars like a les paul, tele, 12 string acoustic, etc and be able to have instantaneous altered tunings, units like Roland VG-8 or VG-88 would be best. They feature Roland 'COSM' system (composite object sound modeling) and are really quite amazing. There are also COSM amps with everything built it like the VGA-7.
Roland VG-8 http://www.rolandus.com/products/productdetails.aspx?ObjectId=442&ParentId=73
Roland VGA-7 http://www.rolandus.com/products/productdetails.aspx?ObjectId=251&ParentId=73
3 - Guitar/Computer Interface
These unit have no sounds built it but convert the 13 pin signals to MIDI and MIDI over USB.
They allow you to control software synths and music notation/TAB type of software on your MAC or PC.
They also allow you to control the sounds in any MIDI keyboard or synth sound module.
Axon AX-50 http://www.axon-technologies.net/modules.php?op=modload&name=News&file=article&sid=36&menu=102