I believe that LW's longslides were designed to be open-top because they were supposedly more reliable. Whether or not this is actually the case, I do not know. The solid top slides, on the other hand, offer more mass for effectively delaying unlock while shooting powerful loads. When I had a longslide G20 back in the day, I had a solid top (it was a slightly different design than the LW's) and I liked it a lot. If I were to order one from LW someday I'd most certainly choose a solid top. As for which is more popular, LW would know.