Is ammonia an acid or a base?
The difference isn't that stark. However, you have to bear in mind the Configuration of electrons on the central nitrogen. We often will draw ammonia with the electron pair atop the nitrogen. That lone pair will readily accept a proton, making it a base. 

To put it another way, ammonia is far more ready to accept a proton than to release one thanks to the lone pair.

Of course, it also depends on the environment it's in. But in general, you'll usually see ammonia (or just nitrogens in compounds) referred to as basic