I've owned a lot of each version, both single and double lever. From a strictly aesthetic view, I much prefer the single. I think it's just more elegant. I do feel the version needs to be appropriate for the vintage of the gun. I wouldn't want a single lever on a post-war gun and wouldn't like a double on a very early gun. Aesthetics aside, I've never had trouble with either slipping or moving though I have had a couple of doubles that weren't timed properly and the levers pointed the wrong direction. I feel they need to point inward, perfectly aligned with one another for the cleanest look. Remember also, there are both steel and aluminum versions of the double, the aluminum being the later.
