If you're building a two player bartop I would recommend a wider control panel regardless of the monitor you choose. Two adults playing on a 20" control panel is way too crowded.
Another thing to consider is the games you will play. Newer arcade (Sega Naomi, Sega Model 3, Sammy, etc), Wii and PS2 games actually use wide screen monitors. If you're using a Raspberry Pi this won't be an issue since it won't be able to play those games.
If you do go with a wide screen monitor you can always use bezels or stretch the screen for 4:3 games.
Good luck with your build.