@timeshifter I agree the Model 3 is much more fun to drive, while the Y seat height and overall human+cargo spaciousness are a lot more comfortable and practical. If the 3 is practical enough, I say make the trade, get the fun car!
Personally I didn't like how the Model Y drove when we tested in 2021, while I instantly connected with the Model 3 and finished each test drive wanting to drive it more.
I think a 2021+ Model 3 with heat pump, double paned front glass, and the newer interior design is worthwhile. Those features may have arrived at slightly different times so check for each individually. The heat pump is really quite efficient and effective, it's a huge benefit over the resistive heating in our old Model S, and our 2021 Model 3 with double pane glass is quieter than every pre-2021 Model 3 I've driven. I also like the 2021+ interior console and door trim changes, though that is just personal preference. Older Model 3's did get real leather on the steering wheel, which is nice, and the very earliest Model 3's had leather seating too. I don't know the cutoff dates for either of those things.
At some point in mid-2021 the front passenger seat adjustable lumbar support was removed. The parts can be physically retrofitted, and there was a helpful thread here for it, but at some point Tesla blocked the controls from working on retrofitted cars in a SW update. (I imagine you could wire up the controls and the lumbar support together through your own microcontroller, bypassing the usual way they interface through the car, but that would be much more involved than just plugging in the parts.)
I think front radar was also removed mid-2021, however my understanding is all AP3/HW3 cars are now using "Vision Only" software even if they came with radar, so to get the better radar-based TACC you'd have to get a significantly older (definitely pre-2021) Model 3 with AP2.5/HW2.5, and also make sure it was never upgraded to HW3.
MCU3 (Atom) vs MCU3 (Ryzen): MCU3 is noticably faster and smoother in the UI. I think MCU2 is just fine though, heck we still get by fine with MCU1 (Tegra) in our S (usable again after its flash memory recall). I definitely see some value in MCU3 over MCU2, but to me it's of lesser importance than the 2021+ improvements (heat pump, double paned front glass, interior console + door trim).
Note there is a driving experience benefit to MCU3: The backup cam is much better with MCU3. Smoother, higher framerate, and quicker to open. With MCU2 it's laggy and stuttery. Even MCU1 in our Model S is
much better at streaming its backup cam than MCU2 in our Model 3. Tesla has improved the MCU2 backup cam with SW updates, it's better than it was, especially in how long it takes to open, but it's still not great and is far behind either MCU3 or MCU1.
When driving forward there's zero driving experience difference between MCU3 vs MCU2 though!
Two features of note came hand-in-hand with MCU3: heated wiper rests and li-ion low voltage battery. Heated wiper rests are handy if you forget to lift your wiper blades in freezing weather. Li-ion low voltage battery is supposed to last the life of the car, or at least many more years than traditional lead-acid, and it's also not supposed to die on you without under voltage warning like lead-acid sometimes do. I consider both of these features nice to have, but not essential. They do add to the overall benefit of MCU3.
There is one notable downside to MCU3: very very few or possibly no MCU3 Model 3's ever came with front USB port data connection to the MCU. Right before the full switch to MCU3 is when Tesla stopped shipping front USB port data connections. That also means the late 2021 MCU2 Model 3's don't have it either. My October 2021 M3P with MCU2 has front USB data but it was among the last to get that. November 2021 onward didn't get front USB data. (Heck many November 2021 Model 3's left the factory without front USB ports at all, just empty holes there, due to parts shortage.)
There are 3rd party retrofit kits to add front USB data, and some folks have had success buying the official parts directly from Tesla, but others have those part orders rejected by Tesla. As the years went by Tesla removed more of the cabling or such involved in the old data path, and so the retrofit is more difficult in the newest Model 3's. Last but not least, Tesla has a track record of blocking use of unofficial retrofitted parts that a car didn't ship with (e.g. passenger lumbar, front radar). As far as I know they haven't blocked this particular unofficial retrofit, but there is risk they could in the future.
Last but not least, Tesla has softened the Model 3 suspension over the years (yes before the Highland). Whether that's good or bad really depends on your preferences. A significant softening happened around beginning of 2021 I believe, such that many folks who went from older Model 3 to a 2021 noticed a difference. Even a 2021 M3P is softer than a older M3LR. I can vouch for the ~ 2021+ softening firsthand.
I've read here that 2021 wasn't the first Model 3 suspension softening, the very earliest Model 3's had an even stiffer sportier suspension, but I'm not sure what the cutoff for that was or how real it even was. Probably no dual motor Model 3 ever had that extra sporty suspension.
In my case I would've preferred the older firmer Model 3 suspension tuning, I was disappointed by the lack of damping control in 2021's (M3P included). I upgraded ours with aftermarket coilovers which gave it vastly improved handling, and surprisingly an overall better ride too. Not a cheap upgrade, but was worth it to me for maximum handling fun.
Good luck!
