This is a tough question, tight yoga pants link or link short flowy skirt. I'm going to say the skirt in general, but it depends on the situation. If you have no butt then the yoga pants are more conservative. If it is a windy day then the yoga pants are more conservative. If you have a bigger butt then the skirt is more conservative.
Skirt. Under the right circumstances, the skirt can be more revealing, but yoga pants reveal every little part of your lower physique's structure. For example, the exact size and shape of each individual butt cheek are clearly defined in yoga pants. This will show under any circumstance in which yoga pants are worn and will attract a lot more stares from guys.
Actually, technically, it wouldn't be the skirt. The skirt has become a feminine idea (or sexy for women) while pants became a masculine idea (yoga pants look better on women of course)...so it would be the yoga pants
I'd go with the skirt. The tight yoga pants show every part of your legs and bum, while the skirt still covers up and leaves it to the imagination. Nobody knows how big or small your thighs and bum are with that skirt.