I have CT Railmasters slung under my M&P9FS and a Ruger SR22P. They are easy to turn on when I want them on, and hold zero certainly well enough for social work. While I would never have my personal defense dependent on a battery, the lasers serve several purposes in my house:
1) My wife can't hold her arms at eye level very long, so she can now shoot with her arms braced from the waist. And I don't need to have my face behind these pistols to aim and fire should circumstances suggest otherwise.
2) When my old eyes can't see the iron sights very well in low-light (like indoor ranges at times), the laser is a godsend.
3) When I want to see if/how my POA varies during a trigger pull, the laser is a *great* training aid. When it dances around the target during my "smooth" pull or in anticipation of recoil, it's not lying.
As for red vs. green lasers . . . pffft . . . if there's enough light to render the red laser useless, then there's plenty of light for me to see the iron sights. I didn't put the lasers on to go against Billy the Kid at High Noon.
The rail, if available, is my preference for a laser mount. YMMV. It doesn't screw with the trigger or grip in any way, and its only on if I want it on.
MY LCP of course does not have a rail. For this I chose the Lasermax over the CT version. For some reason the CT required a mounting screw on the grip front right under my middle finger while the Lasermax mounted without one. This laser is aso only on when I turn it on and makes this little pocket pistol easier to shoot with confidence.
I never intended the lasers as primary sights. Personally, I think this would be a serious mistake. And I can't imagine forgetting to turn it on if I needed it lol . . . after all, it would be fairly dark out

Unless the laser is needed as described above, the only range use it gets is the next to last magazine of the day. This does an operation and accuracy test, and gives me all the practice I feel I need.
Your wants and needs may vary, of course.