TVs are cheaper, but the image quality will be lower than an arcade monitor, or PC Monitor, when used with Composite or S-Video. S-Video and Component video will be better than Composite though.
However, if you have a TV with SCART IN, you can use its SCART connector, as if it were a Standard-Res arcade monitor. In fact, SCART actually turns it into an arcade monitor running native RGB.
Real Standard Res arcade monitors (and SCART TVs), are low-res, but good for MAME, because most arcade games are standard res. Arcade Monitors are more expensive, and may mount better in cabs than TVs, though. Note: these both run at 15KHZ, and you will need to configure your video card to run at the lower scan rate, or use an ArcadeVGA card from Ultimarc.com as it is designed for Arcade Monitor / SCART TV use.
Multisync Arcade monitors (such as the Wells Gardner D9200), can display both PC games and MAME games at their native resolution, and can go up to 640x480 (and in some cases 800x600). With these, an ArcadeVGA or Trident Blade card is recommended. Additionally, you can also use AdvanceMAME to get the right resolution and refresh for each game.
PC CRT Monitors are cheap and readily available, but will not look authentic in MAME. Due to their higher scan rates, you will need hardware stretch for many games. The advantage to these are easy setup, full compatibility with non-emulation applications, and to be able to run windows at high resolutions for PC games, word processing, or other applications. Also, Light guns work well with them.
PC LCD Monitors (Such as flat screens and laptop screens) have fixed resolution, which often results in running hardware stretch for almost every game. Additionally, light guns do not work, either. Additionally, these often have lower color fidelty. The advantages to these are small space, and good for portability, too).