There's some great info here, especially some of what Northwest BBQ told you. If you're a newbie, a few extra thoughts.
1) Consider finishing in the oven or be sure to wrap after you've smoked for a while. Especially if you're serving naked (no sauce), you may be disappointed with dry ribs if you don't do something to keep moist as they finish.
2) I would trim the St. Louis slab as shown, removing the rib tips. DO NOT THROW THEM AWAY. Just smoke them separately and you'll find this helps you to smoke in a reasonable period of time. If you're using a rib rack, the full slab spare ribs will slump over and not cook completely.
3) Consider doing one slab wet, one dry. Many people assume they NEED to have BBQ sauce on ribs until they do it right. I like both, but just because a rib is done "dry" doesn't mean it IS dry or less flavorful.
HAPPY EASTER AND HAPPY BBQ!