Which is the best way in terms of improving server's performance. Buying a new server 30 slot or increase my current 20 slot server to 30?.
Either way shouldn't really have any impact on server performance really. About the only thing you can run into would be things like a game comes out with a newer version which could cause to have you buy a new server entirely. I've had that happen on FS17 and FS19.
I've increased and decreased slots and even done multiple servers for the same game and seen no difference in performance at all.
Great question though!