I'll buy either the PS5 or XB Scarlett near launch. The other one I'll pick up a couple years later. Which one I buy first depends on their prices, their games, and any other interesting factors about the systems that may come to light before launch. I'll note that XB has an advantage in my case in that I have 3 years of Gamepass ahead of me. On the other hand, since Halo is confirmed to be cross-gen, that takes a big motivator for me to by XB off the table.
Thinking about it now, it very well may come down to how much better Halo is on Scarlett vs. XBone.