This will only really save you a meaningful amount of money in the longrun if you traded down and didn't have to pay any money into the deal overall. Neither of your options really seem to give you real savings. As you yourself said, at best, your gas savings by downsaving to the Z would make about 5 loanpayments. So let's not delude ourselves and set the whole concept of saving cash aside for a minute: You simply want something new.
If you just like trading every couple years, get the Z. If you're hoping to keep whatever you get next for awhile, well... If you're been used to the utility and versatility of a pickup, you're more likely to get tired of the Z after a couple years than you would of the grand Cherokee. As you said, the Grand Cherokee provides a better overall platform for a system while having usable space leftover. So that's what I'd go with if I were in your situation.