I came across a Mercs cabinet, and for $50 it was in excellent shape. I liked the look of it too, so it became my MAME project. So consider how they look (i.e. if you like the shape), since you will have to look at it every day.
Also, in my mind many of the old games that I used to play back in the day were vertcial monitors. But now that I have taken up this hobby, I have found that most are actually horizontal (and it seems that my memory is pretty poor). So my cabinet is a vertical monitor, which means most games will not be seen as big as they could be. But in reality, a game originally on a 19" horizontal was only about 15" across. So on my cabinet with a 27" viewable vertical, I will see a horizontal game at about the same size as original, but it will seem smaller because of all the unused monitor.
But I digress... sorry. My point was trying to be that the monitor orientation may be important to you as well. As everyone else has suggested, consider what will fit in the cabinet.