Well, it's little too early to say. I can certainly afford both of them on Day 1 - but there are few factors.
Xbox Series X - most likely Day 1 - only two things not known at this point is the actual pricing and exact launch date. I'm getting this console for BC and some of the games that I missed out in 4k 60fps HDR. (I hope). I've been wanting to get X1X for the same reason for awhile - but at this point, Series X may be the better choice for multiplatform games.
PS5 would be my main console as it's pretty much guaranteed to have more Japanese publisher support - that I tend to go for a lot of Japanese niche games - (unless MS does drastically well - but even the 360 days, they didn't do that well in Japan... so I'm keeping my hopes down). But at this point, I don't hear anything about what new games are coming out, for launch, nor how big of jump of resolution and framerates for BC games. I already have PS4 Pro, and so far it's handling games pretty adequate - so I think I may wait till the games are announced and BC performance enhancements first, as well as the pricing, design etc, before I decide to get it on Day 1. Literally, we just don't know much yet.