IMO, the choice comes down to ribs.
If you like to cook full slabs laid out flat, go with the 22. As Mike points out above, the 22 will fit 3 slabs flat on each of the two racks.
The 18 inch circle is a bit too tight on the horizontal space to do that. Sure you can cook half slabs, use rib racks, roll the racks, bend the racks, cook smaller BBs instead of St. Louis, etc. etc. etc. But the 18 is just too small to do full flat slabs easily. You really can only fit one full St. Louis slab down the middle of each cooking grate, then add two half slabs on the sides.
For my 18, my rib solution was to get a hanging kit from Hunsaker and cook hanging half slabs. Which keeps the lower end of the ribs from being too close to the fire. Works very well and fits a lot.
For everything else, the 18 is going to be way more than adequate. And the 14 probably would be too.