We use Squeeze quite a bit and I've used Procoder a little.

Perhaps it's familiarity, but Sorenson seemed a lot simpler and more elegant to use in its interface, and Canopus, at least when I had it on my computer, which was maybe a year and a half ago, seemed like a memory hog. Sorenson generally works pretty well in the background with other applications running at the same time, while with Canopus I always seemed to run into odd problems if I had a bunch of other stuff running at the same time. Perhaps they've fixed that since then.

The other thing is Sorenson just seemed a little more mature in terms of the batch functionality, the ability to watch a folder, to upload a completed file to a webserver, etc.

The other thing I've heard really good things about is Compressor, which is Mac-only. We are soon switching to Mac for most of our video work and my understanding is that Compressor (which is part of the Final Cut suite) is as fast or faster than Squeeze, so if it's remotely a possiblity in the future to switch platforms, you might try and hold out on buying a Sorenson license.