It is gonna depend on style. It will also depend on what you are doing. Sun dress comes off a bit more classic and fun loving. A skirt comes off sexy. Either should be fine. As for one over the other. If a girl can look good in a sun dress I would prefer that.
Much depends on you and the outfits. Both can look great or one may work better. Try each and see which gets the best reviews from people you can count on. Don't send them cell phone photos. They rarely do justice.
No guy here, but I think they will be attracted to both as long as you can pull it off. Some girls look better in sundresses then in mini skirts. So the sundress is the way to go. On the other hand, some girls pull off the mini skirt look better and therefore, guys will be attracted to that. Depends on the guy too. Most of the guys I know love the mini skirts but then there are some that are big suckers for a girl in a summer dress.
So in whole, try out both looks and see which one you can pull off better and then go with the best one.