It depends on your situation and the cab's location. We have a large family and it's not uncommon to have lots of friends over, so P3 and P4 get used a lot - at least every weekend. P3 gets used more than P4 because there are a lot of fun 3 player games (Rampage and a bunch of Capcom games).
If you have enough people to justify a 4P cab, I think it's important to use a big enough screen to make it comfortable for everyone to see. For extended gaming sessions it's nice not to be packed around a tiny control panel that forces you to rub shoulders for an hour. So that leads to a fairly large cabinet that doesn't look anywhere as nice or classic as a two person cab.
If we were to build another cab we would do a nice clean two player cab - mostly because it would be sitting right next to the 4 player cab. 2P cabs can also be a lot cheaper with half the controls and they don't need such a big screen.