Hmmm... the viewer in exifPro IS much more feature rich than I had realized. I dimly recall someone here posting about its ability to offer multi-pane views so that you can compare photographs. As far as I can tell, Photo Collector doesn't allow this; exifPro will let you view up to four photos this way. As mentioned earlier, exifPro will also allow you to tag, rate and edit the image descriptions which is a nice touch.
Incidentally, tomos, in playing around some more I discovered two things that relate to your comment about going to the start or end of a large number of images from within the viewer in exifPro. Check out this screenshot:

And you'll note that you can accomplish this from the context menu or using the Home and End keys on your keyboard. Also, note that there are buttons to accomplish this as well: |< takes you to the first photo and >| to the last one...
Finally, I had an e-mail exchange with exifPro's developer, Michal Kowalski, who has promised to change the way in which exifPro handles errors during indexing in a future release. He's also agreed to add a couple of suggestions that I had for additional editing features to a list that he keeps, although with no promise that they'll make it into a future version. I'm quite impressed as the reply came within hours of me e-mailing him. Definitely a good sign and one more thing that really makes me think that I want to support development by buying a license. It should be said, in fairness, that Photo Collector's support team has also been very open to suggestions and have been very quick to reply to my e-mails as well.
I'm beginning to suspect that superboyac's assessment:
...from a first glance it seems like ExifPro has more options and customizeability than Photo Collector. On the other hand, I think PC might have more powerful organizational tools, just based on my familiarity with collectorz software. As a viewer, I think ExifPro is the better program, I don't think PC is meant to be used as a viewer.
is closer to the mark than I had originally given him credit for (although I would still argue that while exifPro has a better viewer, Photo Collector's viewer is more than adequate).