Sony will no doubt sell more units in the long run, theres really no way around that, and this is coming from someone who admittedly prefers Xbox, but always buys both.
This idea that the first console to 10m units "wins" is pretty foolish though. If the leaks are true and Sony will only have 7 million or so units ready for launch, and Microsoft has 10m or whatever, does that mean Sony loses the generation because they had less stock at launch and got beat to 10m first? No.
With that said I dont really consider "units sold" an indicator of which console is better overall anyway. It's not a popularity contest. It's about games, hardware, prices/value, and services for me. Whichever console provides the best overall experience, is the one I consider to have "won" the generation. For me, the 360 had the better experience over the PS3, and the PS4 had the better experience over the Xbox One. This generation though, I think it should be pretty close between the PS5 and XSX, in terms of the overall experience.
I like what I'm hearing from both camps in terms of the factors that actually matter to the average gamer. Not the financial bottom line, which doesnt effect me at all.