I vote for pork butt for the first smoke, and here's why. You can put a temp probe in a butt. Since you can't probe ribs, the temptation for someone at their first smoke is to keep opening the door to "look" at the ribs. And just open the door from the sheer excitement of it all! It seems like I see a lot of posts where people do ribs for their first smoke, and are disappointed for one reason or another. But usually because they are undercooked, because they open the door to look, spritz or mop (not necessary with the SI), sauce, wrap, etc. and just general unnecessary fussing. And just cuz they want to see what's going on in there! Since ribs go by time only, you DO have to open the door eventually to check them for doneness. Pork butt is a nice long smoke, and goes by temperature only. When it's done, it's done. Open the door and take it out with confidence. It's also very forgiving, stays moist, and is practically fool-proof. And it further seasons the smoker nicely.
Welcome from a fellow Wisconsinite by the way! I use my SI all winter long!
