Shotgun,
I'll share some of my thoughts about a weapon mounted light, handgun that is. For a while I was a gung ho tactical-light-on-the-gun guy. I wound up with a Surefire X200 that is my pick, but I haven't got to try a M3X. All my guns had to have rails, I even built a 1911 from a Caspian slide and frame with rails. Used them in training at Gunsite, Thunder Ranch, and Blackwater - they worked great - free hand to work doors and all that.
One day as I pouted over my H&K USP for having some kind of H&K proprietory rail that my tact lights wouldn't fit, I suddenly realized I never carry a tact light with me or on the gun even though I have a Blade-tech holster for a Glock with light and another for a 1911 with light. They're just too big. S I began to re-evaluate my light strategy and here's I found. I always have a handheld light with me and I can't carry a rail light nearly as easily. So if "it" happens, I'll have a handheld for sure so I'd better be able to use it. And that's my final answer.
But at home, overnight, the nightstand gun with a tactical light is a good idea. Tangle, my dog, and I were awaken one night by a threatening sound. When I saw Tangle barking her head off pointing at the doorway, I knew this wasn't the usual "bump in the night".
I was remembering what Clint Smith told us in lecture about inmates teaching inmates to break into a house and rush to the bedroom to catch the occupants before they could respond. I retrieved the C&L'd 1911 from the bed stand and pointed at the bedroom doorway. To my pleasant surprise the WML (weapon mounted light) was already on and flooding the bedroom doorway with blinding light. This was one time I carefully and thoughtfully considered calling the police. But I didn't - dumb, dumb, dumb - and I proceeded to follow dumb with foolish and "cleared" my house myself - dumb, dumb, dumb! I was very fortunate that it wasn't an intrusion, and maybe "they" were too. Never did discover what the noise was.
The next morning I "debriefed" myself on my response and realized how awkward and time consuming it could have been to have to retrieve my gun, with one hand, and try to retrieve a light with the other hand, get the gun and the light indexed in the same direction, etc. So having the tactical light was a distinct advantage. Clearing my house was suicide - what was I thinking!
So my strategy is handheld light while out and about, WML overnight. But if you choose a handheld, practice with it and practice looking around both left and right corners.
Lastly, a handheld needs a lanyard. There is no way to hold a light in one hand, a gun in the other hand and open a door with the other hand - we don't have that many hands! You could holster the gun, open the door and hope you can get your gun back out before you need it. You could put your light under your arm or in your pocket but you've got a long reach to get the light back and you could drop it. The lanyard allows you to drop the light to open the door and easily retrieve it. The lanyard also solves the dropped light problem.
Then having said that, I don't carry a lanyard because it draws too much attention to my light. Anyway that's my thoughts and I've said too much again! Why didn't somebody stop me!
You do well to consider your lighting needs!