As mummel said, the costco ribs I've purchased lately had the membrane removed already so make sure you're not removing the skin under the membrane. Once you've done it, you'll know the difference. The membrane is super tough, think a paper thin rubber band that can rip if you don't pull it correctly. I've yet to try the 3-2-1 or the 2-2-1 method. I've always just done mine completely in the smoker.
Baby backs have less fat and if you have "finicky" eaters they WILL eat the entire rib. Spare ribs have more meat, and more fat also, they have the end of the bone where there's a knotty mixture of fat, bone and cartilage that those "finicky" eaters will just throw away.
I know everyone has their own opinion on rub/sauce/etc, but I usually follow the "best ribs in the universe" aka BRITU method and they are awesome. It may be my taste buds, but I use 1/2 the cumin called for in the recipe, that's my only change.
Oh and ALWAYS cook them bone down or standing up, either in rib racks or coiled with a skewer holding them. Oil will pool up inside the ribs like a giant cup if you cook them bone up... I made that mistake once.