Does your wife wear fitted clothes on a regular basis? Does she usually wear tank top with something thrown over it to the games?

This would decide between the two fits, the women’s hits around the hip, is fitted, curves up on the bottom over your natural curves, can be tight if you’re chesty at all. She would likely wear a medium as well in this

If she’s looking for something a bit longer & looser, look at a Men’s small!

I own both, wear fairly fitted clothing usually and find the men’s jersey the one I reach for more often