I have a rail mount laser, that I can swap between several pistols, but I only use it for Snap-Cap practice. I wouldn't waste time chasing a little red dot around in a self defense situation.
For home defense, where the pistol isn't going to be in a holster, a rail mounted light might serve you better. I agree with the S&W micro90. It's ambidextrous, with either constant on, or intermittent on capability.
With larger (Full-Sized) pistols, the instant on switches could be hard to reach, but with the PF-9 (or even my XDM9c), they're easy to tap.