i just believe it will be easier to maintain visual contact w/ a sight thats not moving as much. there is no way you can keep focus on a sight on the slide during recoil, therefore everytime the slide goes back into battery you have a very brief moment when you are refocusing on it. may not amount to much, and there are probabably other variables that will would effect follow up shots more, but thats my .02 take it or laugh at it and leave it.
Frame mounted sights = the older way
Slide mounted sights = the new school way
The truth is, its only recently that sights were small and light enough <as well as resilient enough> to be mounted on the slide.
The slide is moving, very VERY fast, I would not worry about refocusing or even the perception of the slide moving. It works for me anyway. I would say, mount it on the slide.