I was also interested in the keyboard, and I think they are either identical-feeling or nearly-so across all three models. The new MBPs could be a tiny bit more resistant, but that could be my imagination (or the fact that the other demo machines have been in use for months, and therefore a little less springy).
I didn't notice a huge difference in screen brightness/resolution/responsiveness, though I wasn't looking that hard.
For me, the biggest upgrade in the new MBPs is the ports, since I'm on a 2-port MBP from 2017. If I could get SD/HDMI/MagSafe on an MBA or the 13" MBP, I would do so in a heartbeat. The speed of the new MBPs is amazing, but I (a non-technical founder) could easily get by on an M1 MBA.
I talked with a knowledgeable Apple employee, and he said that he expects they'll keep the 13" MBP around, and that when it updates it should get the same ports (and definitely MagSafe, which effectively gives you another port) as the new MBPs. He didn't expect the MBAs to get anything other than MagSafe.
My plan for now is to try to hold out until the 13" MBP is updated and get one of those. I prefer the smaller size, and the cost of the new MBPs is just not justifiable for me. I might even get an MBA if it gets MagSafe + 2 USB-C. One interesting tidbit is that Apple's tech specs show longer battery life on web browsing for the MBA than the MBPs. That metric is a relevant one for me. AppleCare is also significantly cheaper on the MBAs than MBPs.