Here are a couple more photos.
(http://farm9.staticflickr.com/8111/8499740797_0b4d463c98.jpg) (http://www.flickr.com/photos/mashby/8499740797/)
Key Cap Comparison - Tops (http://www.flickr.com/photos/mashby/8499740797/) by Michael Ashby (http://www.flickr.com/people/mashby/), on Flickr
(http://farm9.staticflickr.com/8506/8500847516_ff917b5b97.jpg) (http://www.flickr.com/photos/mashby/8500847516/)
Key Cap Comparison - Bottom (http://www.flickr.com/photos/mashby/8500847516/) by Michael Ashby (http://www.flickr.com/people/mashby/), on Flickr
(http://farm9.staticflickr.com/8371/8499741207_c49acc1376.jpg) (http://www.flickr.com/photos/mashby/8499741207/)
ABS & PBT Side-By-Side (http://www.flickr.com/photos/mashby/8499741207/) by Michael Ashby (http://www.flickr.com/people/mashby/), on Flickr
(http://farm9.staticflickr.com/8519/8499763287_d22f15c6bd.jpg) (http://www.flickr.com/photos/mashby/8499763287/)
Vortex PBT vs. IMSTO's Thick PBT (http://www.flickr.com/photos/mashby/8499763287/) by Michael Ashby (http://www.flickr.com/people/mashby/), on Flickr
My pictures aren't the greatest, but looking at them in person I'd have to say that the key caps match the standard Vortex OEM profile -- both in profile and in thickness. It is thicker than the Signature Plastics key cap, but not as thick as IMSTO's thick PBT.
That being said, the Vortex PBT caps do have a lower pitch when bottoming out than the Vortex ABS. They may be the same thickness, but they have a "thicker" sound. Unfortunately, I don't have a full set of IMSTO's Thick PBT to do a proper comparison. :-[
Also, if I'm not mistaken, the Vortex use the OEM profile, where as IMSTO's are Cherry, so your preference in profiles may tip your decision one way or the other. ;D