This will really depend on your market. Pruning vs. removals, size of your removals, how much crane work (if any), size of the tow vehicle (gcvwr and chip capacity) and your crew size. Also look ahead, what is the market going to do. We are facing the beginning stages of EAB so big nasty removals are coming for sure. What stage are you in? Has it passed through and now you are just in maintenance mode.
But if you have been operating with an 18” chipper.... you will probably hate life going back to a 15” as your big chipper, assuming you would keep the 12”. With machine feeding, a solid tray (probably like your morbark) is a must. Our folding tray requires extra care and finesse not to wreck it.