If you can afford it, an Arcade Monitor will give you the best picture but it is often limited in resolution and therefore using it for PC Applications will be limited in fucntionality because of this. Console Emulators will be fine, though (they run at low res). You will need for this a video card that can output at 15Khz (CGA) resolution such as Ultimarc's ArcadeVGA or a software tool like Soft15Khz.
A TV will give you a bit lower quality picture but it can be OK if you just run arcade type raster games. TVs can made small text hard to read, however larger text and most graphics do fine. When using a TV, the input is what matters. Using SCART RGB (European Standard) will give you an authentic arcade picture. TV output using component video is usually very good. S-Video is acceptable in many cases, too. Using composite or RF Modulation usually results in a very low picture quality. Using an HDTV may or may not improve quality as they have to scale low res games to higher resolution, and may be a different aspect ratio (16:9 instead of 4:3).
Using a PC Monitor will be easiest to setup due to easy connectivity and no 15Khz Hassles. However, they will not give you an arcade real picture due to smaller dot pitch and scaling low res games to higher resolutions. You may want to consider a PC Monitor if you want to play higher res PC games on your cabinet or vector games on it.
There are also some monitors that do multiple resolutions, allowing both arcade resolutions and allow you to run windows natively up to 640x480, or in some models 800x600 or even 1024x768. Some of these include the Betson Multi-Sync, Billabs BL2C0P, Nieman Tri-Sync, and others. These are expensive but have the benefit of running at multiple resolutions. You will still need a 15Khz capable video card to run at Arcade Resolutions, though.