The answer really depends on what you are going for.
I have a showcase and I love it for what I have in there ... Virtua Tennis 2 and Sports Jam. A golf game (VG,EAG) would be good in there too.
For classics, however, I prefer a more intimate setting, huddled in close to the laid-back monitor. Playing Galaga, Pac-Man or Robotron (yes, I know that Robotron did not have a laid-back monitor, but it is a good example of a game where the experience is different once you back up a few feet) on a showcase will never be the same experience as on a smaller, closer cabinet. It is tough to recreate that "lone saviour of the universe" feeling of Defender when you're four feet from the monitor and people are standing all around you.
If you loved four-player games, then a showcase may be right for you, particularly if you and your friends have expanded since youth. It is bloody hard to fit four guys playing TMNT around a Galaga cabinet.
At the end of the day, it is up to you as to what you want to create or recreate. Think about what you loved about the games you played and build to that.
Cheers.