I want to purchase a window air conditioner. Some say that a 220 vac air conditioner is best, I want to know why.
I understand that with the 220vac system, you can have more capacity, more BTU's. I don't need much since the room is small.
What the difference?

Typically, 220 will be more efficient in larger rooms... but but to about 10,000 BTU, they'll generally be 110.

Here's a typical room size/btu/volt guideline:
http://www.feddersoutlet.com/sizingguide2.html