I will have to stay tuned for this one, since I am also looking to gather more info on these sites.
From my cursory inspections, COMC seems to have the best selection, but I am still confused on some of their shipping pricing/policies/practices. I might shop here for higher valued cards, but as for commons, or even most parallels, their lowest pricing seems to be a little above what I would think of as 'fair'.
sportlots seems to be better pricing, and from what I've read, better shipping potentially, but it seems a lot of listings lack pictures/scans so it might be a bit more of a shot in the dark. I would potentially shop here for commons, but not much more pending individual sellers having enough info and/or photos
As for Beckett Marketplace, they seem to have quite the selection, but I've yet to come across any 'good' shipping deals to Canada, although many of the bigger sellers seem to have really good deals on shipping to the US. Once this is all over, this may be an option for me to consider as I am only an hour from the border. In future, I may end up offering to coordinate runs over the border with other members if arangements can be made.
As for Burbank, I think I read somewhere that they are no longer shipping here, but that may have changed.
Anyhow, hope this helps, and/or gets a bump up for more reads and input!