A PC monitor has the advantages that it's perfectly simple to set up with Mame. The biggest drawback is that it using a computer monitor doesn't necessarily look or feel like you're in an arcade. Much less importantly, you can get a very large (25" or more) arcade monitor for a comparable price to a much smaller computer monitor.
An arcade monitor can be a pain to set up (but it's a LOT easier now with the new ArcadeVGA video card from Ultimarc). An arcade monitor provides the heart lifting feeling of watching the arcade cabinet and not being able to distinguish it from the actual arcade game it's emulating. I really don't think that you can beat that.
For me, the choice to go with an arcade monitor is the same choice that I made to not be satisfied playing Mame on a keyboard or with gamepads or while sitting at my workdesk.
Probably everyone likes their decision best (be it a TV, PC monitor, arcade monitor, projector, vector monitor, or a vector projector) and would say the same thing, but for me it's my way or the highway - I can't imagine going any other way than using an arcade monitor.