I guess I'm going to go against the crowd here a little, but I would go with the rotating 21" CRT. It's obvious from your posts that either maximizing the utilization of the monitor, or viewing the games in their original aspect ratio/resolution is important to you, which would suggest going with a rotating setup. And once you have decided to go with a rotating setup, it's really not that much more difficult to build a rig to support a 21" CRT than anything else.
(For the record, my cab has a 21" rotating CRT in it, so maybe I'm biased

It's been working for over four years with no problems)
As far as LCD's go, I actually like them more than CRTs for regular computer use, but to me the classic arcade games just don't look the same on an LCD.
Koz