PermissionsEX is the better of the two in my opinion, however it is an abandoned plugin (meaning it won’t be updated again) so I suggest that you learn LuckPerms because that seems to be the main one now. It’s really just up to personal preference, try both and make your own choice on which one you prefer.
If you need a tutorial for LuckPerms, you can find one here! If you have any future questions post them in Community Support and people will be able to help