Generally speaking, you never want to use a low port (<1024) for anything other than the service assigned to it, because it causes all kinds of headache. Both on your side and on the other side. As for high ports, pick whichever one you prefer. They don’t have any binding to a given service, though there are some conventions.
The thing that shows people you’re running a VPN is not the port but the protocol header, so changing the port is pretty much useless if you want your ISP to not know you’re running a VPN for some reason.