I've only ever used 2 monitors with my DOS computers(one CRT, one TFT), but I would personally go for a CRT. Haven't gotten very good results with a TFT monitor. Here's a little rundown of what I experienced with the monitors in question:
CRT(a 13" Macintosh monitor): Very sharp image with nice colors, though a little bit dark. CRTs are what I believe to be the best monitors to use with DOS computers.
TFT(a 24" Samsung SyncMaster 224T): Typically very blurry and with very dull colors depending on the graphics card(only the Matrox Millennium II looks good on this monitor), the screen is off-center with no way of adjusting it(too far up and too far to the left), and as this is a 16:10 monitor, the image is stretched horizontally. It doesn't look bad, but this is not how anything running under DOS was meant to look, can sometimes take a while to pick up the video signal.
Personally, I like using my DOS computers on a 27" flat-screen CRT TV with S-Video, be it either with an external VGA to S-Video converter or the S-Video output of the graphics card(preferably on an ATi graphics card; nVidia graphics cards seem blurry and sometimes have a blue tint in the video and cut off part of the output). It looks pretty good, but will never match the picture you get on an actual computer monitor.
Creator of The Many Sounds of:, a collection of various DOS games played using different sound cards.