15" - more rail space for activities, at the expense of additional weight
13" - less rail space for activities, with the benefit of less weight
There's also a component of aesthetics, some people like one look over the other.
Depending on the gas system length you may gain further benefit of protecting the gas block from impact, but on a 16" gun you're most likely going with mid-length or carbine gas, so both would cover the gas block.