Sportlots is more work on the part of the seller, as you have to do all your own shipping and inventory control, but is great for unloading commons.
Check Out My Cards is consignment, so depending on how much you're willing to pay per card for faster processing into their system, seems to work better for higher end cards and visually is a much nicer system.
Ultimately, both end up being cheaper to deal with than eBay, but of course, you end up with less traffic, though both sites get a fair bit.
On Check out My Cards, when I first put my cards on the site I sold a ton right away because users can search by newly listed and get the great deals. Now I'm down to about 40 cards and haven't sold one in about a month.
Plus, if 10 people are selling the same card at the same price it seems that the one with the larger collection gets listed first.
Even with those negatives BUYING cards on the site light years in front of ebay!!