SMBs come in a few varieties
1) Open SMBs are the most simple and thus cheapest ones, they have an open bottom and are easy to fill, this means when they hit the surface post deployment they could loose some air if not controlled
2) Semi Closed SMBs have a funnel and hold the air in, so they don't go limp on the surface
3) Closed SMBs are the nicer variety, they often have an OPV (over pressure valve) and an inlet nipple, the more luxurious ones have a non return valve on the nipple instead of a push in action

Reels come in two main varieties, a ratchet reel mainly for deep SMB deployment and the standard reel for cave & wreck line laying, this version does not have a ratchet and locks with a thumb screw.

Spools, well are just spools, they come in plastic, ABS, Delrin, and Aluminum. It's best to avoid the injection moulded plastic as they break easily. It's also important you don't buy a spool with too much line on it or it will obscure the holes. If you are using thick gloves you might want a larger finger hole.