It's preference really. The Stationary mount goes on the frame, and can be removed quickly and easily. The biggest con is this kind of mount won't fit in most holsters. If you're removing the rear sights and mounting it to the slide, this is much more low-profile and less likely to get caught on anything, and the Glock will still fit in most holsters as long as they don't go up to the rear sight (like the big duty holsters do). The disadvantage is you need to remove the rear sight, which may require a gunsmith (depends on how confident you are at the idea of tapping out your rear sight), and is not easily reversible.