Despite all the arguing over them, launch libraries are largely meaningless. Also, the only real launch-day killer app on either PlayStation or Xbox in the entire history of either brand was Halo CE on the OXbox. There was nothing else that really stood the test of time as one of the all-time greats or even as a simply game that pushed the limits of the hardware it ran on. There were some decent games to be had on all of them, but nothing so groundbreaking, revolutionary, or just plain good to make most people think "Wow! I have to get this system on day one to play this game!" The only brand to consistently have some major must-have game at launch has been Nintendo (and even then only their home consoles, Switch included).
What matters is the long term, and in that regards it's a marathon, not a sprint. Both systems will have excellent libraries, more than enough to justify a purchase of either one. I've bought/picked up three systems on launch day (soon to be four once the PS5 is released). I didn't get them for the games that were released for it that day. I bought them for what I expected to be getting in the months and years ahead. I like to get at least one system from a given generation fairly early, plus there was no telling if stock would be a persistent long-term issue for the ones I've gotten day one, so I decided to preorder. It had nothing to do with the launch titles available.