PS5 controller for PC?

Can I use the PS5 controller to play games such as RDR2 on the PC?

Or would it be more worthwhile to get an xbox controller (the newest)?

Yes, but you have to download a program.

It is not the case with the Xbox controller, there you can simply connect your Xbox controller to Bluetooth and start playing.

Can I use the Xbox Series X controller?

That would be new with the PS5. The PS4 can also be used without it.