I believe your decision should be based, in part,  on what you plan to have your MAME rig do.  I initially had a 32" TV planned as my monitor.  It worked great for the MAME arcade games, but when I wanted to have the nice crisp look of pinball tables at higher resultions, I realized the TV was the Achilles heel.  I decided to go with a 19" CRT monitor, although I wish I had a larger monitor.  Personally I prefer the ability to have the higher resolutions as it gives more flexability for your MAME cab.