I have one of their skirts and one GymGirl by SkirtSports. Both are the type with compression shorts under the skirt. I like my RunningSkirt skirt better than my SkirtSports one; it has a waistband and doesn't ride as low as the SkirtSports one, so I find it to be more flattering, and it feels like it stays in place better. It also is just overall a "cuter" style, if that's important to you.

The compression shorts ride up on me in both skirts (hence why I'm trying the short-less one for my new one) but I find that once I start sweating, they stay in place better. I also sometimes use Body Glide on my thighs and it seems to help make the shorts stick.
I tried on both the Running Skirts styles at the last race expo I was at. The big difference between the two (besides the obvious shorts) is that the version with the briefs is an inch or so shorter than the version with the shorts. I felt a bit "exposed" in the short-less version, so I originally went with the athletic skirt with the shorts.
They seem to run true to size; I take a women's size 6 in pants and the twin that was helping me at the expo (I'm not sure whether it was Christy or Cindy) recommended I wear the size 1 in the athletic skirt or the size 2 in the regular skirt; both of these sizes are marked as being a woman's 4-6. If I had been choosing sizes myself I probably would have sized up to the next size (because I'm certainly not a 4!) but the recommended size fits me well. So I'd say stick with your dress size when choosing your skirt size; the material has a lot of stretch to it!
I love my running skirt and have retired all my running shorts to the back of the closet. They're more comfortable and I find they cover a lot more than a typical pair of running shorts! I do prefer my running capris, but in the summer when it's too hot for the capris, the skirts win!