My green lasers appear much brighter than my red lasers in and around my house. I can also (with difficulty) see the green laser on a target in daylight, can't do that with a civilian red one (not that I would even rely on one for daylight use).
I have also read that the green ones are worse for your eyes and can cause blindness even at large distances.
There are many types of lasers, from units that turn on when drawn from a holster to pressure switch activation when you grip/press them to buttons you push to turn on. All depends on what you are looking for and especially size and features. Do you want it paired with a flashlight? Some of the grip units for a 1911 (example) are the best because they work with normal holsters, most others need special holsters.