I'm about to try out gPodder myself (been using Canola's podcast function). Let's see..differences of the top of my head.

Well Canola has canola-tuning so you can get podcasts without having to manually enter the url or copy and pasting urls. Just click the feed button and click canola (podcast for podcasts). Not sure if that's the case for gPodder.

However Canola cannot do video podcasts yet. Well technically it can do video podcasts, there just isn't any support if you have problems since your not suppose to use the audio podcasting area for videos (video podcasting is coming).

I'm going try out gPodder since I want something that will hopefully work better with video podcasts. If so I'll just use gPodder for video podcasts.