What's the performance difference between a front-load and top-load bike stem?
There should be no difference, other than the quality of the materials and workmanship.  One can clamp just as tightly as the other, and both will be apt to produce the same symptoms if the bar is slightly undersized or too flimsy at the mount point.  In both cases, if you do have a slightly-undersized bar you want to keep you can grind down the faces of the clamp a hair (though it would tend to deface the upper unit more).

I suspect the main difference between the above units is weight and "sex appeal" -- the upper unit appears likely to be a few grams lighter, and it's that cool black color.