For a while, Model S's were actually more expensive on the used market than they were new. People didn't want to wait in line six months for a new one, so people peddling their used ones could pretty much ask for the sun and moon for theirs. Doubtful the same will happen with the Model 3, but at least for a while, you might be surprised how little you'll save over buying used.
And don't forget the Model 3 has another trunk in front, which looks to have a much larger opening. You're not going to carry a sheet of plywood in there, but it should be much easier to get a big suitcase in front than in back.