The girls and the guys shirts really matter not. We know this. If a girl is too big for girl sizes she's going to go for the guy shirts, that's common sense. Sadly though it's those guys mediums which always run out, followed by guys small.

On the other end of the spectrum, people should know that kids sizes can be comparable to girly sizes, so if you buy for kids and something sells out, anything above an 8 should be comparable in a girls size. This also works well if you are looking for a kid and there isn't a kids size section. I believe girls sizes are slightly longer, but some people might prefer that.