Just for reference (mine) what are you using the rPi for? I have two rPi 3b+ and 1 rPi Zero running currently;
rPi 3b+ running Diet-Pi hosting APCups, CUPS, and Unifi Controller
rPi 3b+ running Raspbian for HamRadio activies (using Build-A-Pi found on YouTube)
rPi Zero - DMR radio hotspot (transceiver to internet gateway)
Still could use one or two more
I would go with the rPi 4 as a desktop... bit more power and more memory..
That's the first model that offers a real desktop option. No one has suggested trying anything else.