Getting a base 16" (either the 2019 one or whatever last Intel one may still come end of this year) would basically just be to hold me over to ride out and observe the transition. I don't want to ride out the transition on a 2013 MBP on its last legs that could call it quits any second now, but i don't want to commit long term to a spec-ed out Intel MBP right now either when i know i want to make the ARM jump in the near-mid future. So going low spec 16" Intel MBP and then trading in before it loses too much value would give me the freedom comfortably wait out the transition period and then make the jump whenever i was ready on my own time but also without taking much of a loss financially on th trade in because it'd be a lower starting price (and buying refurb may counteract that loss anyway).
My honest advice on this one: If you're only buying this thing to last you comfortably through the ARM transition, look for a 2015 MacBook Pro on eBay. If it's the 13", then you've got Broadwell, fast RAM, fast and removable SSDs. If it's the 15", then you've got decent AMD graphics, a super-fast SSD, and if it's eligible for that battery recall, then you just send it to Apple for a day or two and it'll come back to you with a brand new keyboard, mouse, trackpad, and battery for free! Your rate of devaluation won't change, you'll just be spending less money to tide you over as, specs-wise, both of those Macs are plenty good even still.
Hell, you may even want to keep it for backwards compatibility seeing as, ports-wise, it's a very different machine.
With your scenario, wildly speculating here, but what if the 13" and 16" ARM MBPs are the same specs and the main choice a buyer is making is just down to screen size and maybe amount of USB-C ports? The main reason the Intel 16" is more powerful than the 13" is because thermally they can't put the same CPU/GPU chips in both form factors and have good performance/battery life in the smaller MBP. But with ARM chip efficiency and the potential to not need active cooling, those thermal limitations may be a non-issue and differentiating the two sized MBPs via performance may not even be necessary. That'd be interesting... Realistically though it'll be differentiated via core count most likely.
Honestly, that seems pretty plausible. Intel's offerings and Apple's thermal envelope were all that determined the performance disparities between the 13" Mac offerings and their 15"/16"/17" counterparts. In fact, if Apple had made the 16" MacBook Pro thicker, they could've fit a mobile Xeon processor in there with a decent Quadro or AMD Pro GPU.
There was such a disparity on the PowerBook G4 side of things, but, as I recall, it was nowhere near that substantial. You might've gotten a slightly slower G4 processor or a slightly weaker video card on the 12" than what you'd see on the 15" or 17" at that time. But it was never to the degree that you have with any of the 13" MacBook Pros when compared to their contemporary 15"/16" counterparts.
So, yeah, I would think that, unless Apple is pushing these chips to the point of the limitations of their current thermal envelopes for both enclosures, there'd be no reason other than marketing to not keep them the same. After all, there's no noticeable speed disparity between either the 2018 or 2020 11" iPad Pro and its 12.9" counterpart.
Then again, I'm sure that 16" MacBook Pro customers would appreciate Apple not holding back on the CPU for that machine just for the sake of parity.