I'm kind of knocking around the same general concept, but in my case, it's a price comparable Series 4 Stainless (black), GPS+Cellular, vs. a Series 5 Sport/Aluminum (gray) GPS. Same prices, either would have a near complete factory warranty and/or AC+ added (so it's not like comparing a no-warranty device).
Kind of leaning towards the SS model, it would be something different from my existing Sport model, and I've got a couple of use cases where having cellular on the Watch would be super handy.