I did only spares for the first year I had my WSM. Then Costco had some Baby Backs at a real good price. I gave them a try and haven't gone back.
I find the spares to be leaner and sweeter, with plenty of meat. Because they are a bit easier to dry out, I usually cook them at 250 for 4 hours, while I cook spares at 225 for 6 hours or so.
This weekend I cooked three racks of Baby Backs on my top rack, and six turkey legs on the bottom rack. Everything took about four ours, and my family couldn't decide which they liked best! I think I've finally mastered the turkey leg so that it tastes like the ones in the amusement parks - and I think the drippings from the ribs added a nice flavor.