The Galaga cab may be more boring than the Galaxian one, but the crowd pleaser is definitely Galaga. Power-ups, I love Galaxian, but it's just too slow in my opinion. It also doesn't switch-up gameplay like Galaga does when you get double-ships and bonus rounds. For those that say Galaga is too easy, wait until you get into the higher stages and say that. Unless you're an expert, it's not that easy to maneuver through bullets and diving ships with two ships of your own on the bottom of the screen.
You want the one people will play the most... IMHO get Galaga.