It's down to the expected use case.
If you have some reason to want portability, like you travel for work or expect to want to game at a place other than you home, then a laptop is likely the right choice.
If you only expect to game at home and don't have a need to constantly move your system around, a desktop is usually a better "bang for the buck".
Personally, I don't travel and don't have a need to move my gaming rig around. I also like having the ability to upgrade in a piecemeal fashion. So, I have a desktop. This particular PC of Theseus has been going for a decade and a half now and shows no sign of stopping.