What's the difference in attendance at an all women's health club/gym and a regular health club?

Depends: Some women do not feel comfortable around men and feel more focused at an all women's gym. That gym may also offer more female oriented classes as well. I personally think it does not make a difference. The proximity of the gym to your home, the hours, the cost are more likely to affect long term regular visit. Visit them and pick the one that you like...That's the most important thing.