Assuming I don't upgrade my pc and go back to consoles, I would buy an XBOX day one and maybe a PS5 down the line.
While I am really interested in what exclusive devs will do with the SSD, Sony's first-party games generally do nothing for me (Spiderman and Ratchet and Clank being exceptions). I'm sure third party games won't see much of a difference between both systems so owning either would be fine. What it then boils down to is two reasons:
First, I'm already way more tied into Microsoft's ecosystem. Digital purchases of games and movies going back to the 360 plus og XBOX games and having that all available day one. From what I have seen there is some confusion surrounding Sony's BC efforts and that it may take a while for them to enable more games playable. This feature is important to me because when I moved over to PC I left a backlog of games on both. And while I checked off a few of those titles on PC, if I were to go back to consoles, I would like to continue chipping away at that list.
Second, the controller. The single biggest reason I could not get into PS4 as the main console was the controller. The position of the sticks is horrible for my thumbs in shooters. My right thumb would constantly slip off the stick. If I reposition my grip to stop slipping, it would just result in pain. I would like it way more if the sticks and dpad/buttons were swapped. To be fair, I would also like to see this on the xbox controller for comfort reasons but at least because of the way the stick is shaped the tip of my thumb catches the edge and doesn't slip so I at least maintain some control.
Either way, I think both will be great devices. For now, though, PS5 will need some exclusive games that appeal to me plus a guarantee that my backlog will be playable before I consider it.