I'm still looking around at them (and trying to convice the wife to not be mad when I spend the money), so this is more from a researcher's standpoint rather than a user.
Green is brighter, but due to a higher energy cost to create the green laser, batteries last but a fraction of the time that a red laser lasts. They also tended to be physically larger (again due to the necessary engineering to create the wavelength needed). Crimson Trace believes that they've found a way to solve both of these issues and are coming out with a green variant of their LaserGuard soon. They've also announced that they're working on green version of the grips, as well.
I've used my brother's Glock 17 with his grips on. Red laser, simple to use. If you've got a positive grip on the weapon, you're triggering the laser. It's actually a small button just under the trigger guard on the grip that you push with your middle finger when you grip it. No switches to get bumped on when using.
It's what I'm looking to get for my XD9. The LaserGuard is good, too, but it does have that external switch that you have to flip on.