you can easily fit uncut slabs babybacks(or spares for that matter on the 18.5"
2 down low, and 2 up top. Just curve them to follow the bend of the smoker on the oustide of the racks. Large bone portion of the ribs to the outside.
typically i do 3 at a time, start 1 down low, and 2 up top, and after about 2 hours move the bottom rack to join its buddies on the top rack.
With rib racks you can do alot more, i had a buddy do 10 slabs on the 18.5 using rib racks. 4 down low, and 6 up top.
good luck with your decision.