Glad I stumbled upon this thread. I'm in essentially the exact same spot right now: wavering between a used 2015 Model S P90DL (AP1, pre-facelift) with just under 40k miles for $47k or a base model Y for ~$51k (not including other fees). Prices between the two are relatively even, so I can see why it's a common decision for prospective buyers right now.
For me, the big trade-off seems to be that the S has exhilarating acceleration and handling, but it has dated electronics, no warranty, and inevitable problems to be fixed (like with the MCU, I assume). Plus the particular one that I'm looking at is currently 100% dead in the dealership's parking lot, which isn't exactly a confidence-builder (means it was neglected so long that the battery drained out). Meanwhile, the Y is more boring (I know I don't need to go 0-60 in 2.8 seconds, but I can't deny it would be a fun option to have), but it's undoubtedly more practical with its increased storage space, better range, current electronics, and warranty. Plus, with a new Y, I would be the original owner, which is better than buying a used P90DL that might have been abused by its previous owner.
I've heard that model S owners love their cars and find them fun to drive. But then you get a few people with horrible experiences who visit the shop way too often, and it kind of scares me. But when I put it all in perspective, it seems like S owners are generally happy with their cars. Some folks just wind up with lemons, which sucks, but it could happen to any model of Tesla. Meanwhile, the Y is an untested product, and it's in its first few months of production. That first model year might have some kinks to iron out.
I didn't give you an answer, but maybe hearing some thoughts from someone making the same decision might help. Let me know what you plan on doing and why.
And if anyone else wants to weigh in on my wavering between the 2015 model S P90DL and the base Y, I'm all ears.