My advise would be; if you have the funds and the need now, then get it now. You've been going 5 years on 8GB RAM, and so for dev purposes you'll notice a good

My advise would be; if you have the funds and the need now, then get it now. You've been going 5 years on 8GB RAM, and so for dev purposes you'll notice a good improvement going to a machine with 16 or 32GB RAM. If you can, get more RAM and make the machine last. I'm quite excited about ARM Macs, and I suspect they will be excellent in many ways, but there is always new hardware on the horizon. If you keep waiting for the next promise, you'll be suffering in the mean time. Get a decent Mac now, and get another one in 5 or 6 years time.