I have used the ITI M3 and Surefire Nitrolon (P116D & P116D-DG) on the Beretta M9. Information as to your intended purpose would have been helpful in providing relevant information.
All three can be had with adapters that allow the mounting of the light on a pistol without a rail. The adapter came with the Surefire lights and needed to be purchased in addition to the M3. The adapters take 5-10 minutes to install and usually prevent pistol from being holstered in a normal holster.
Of the three, the worst of the lot was the ITI M3. (It was also the least expensive) On multiple samples the bulbs failed to hold-up to extended firing sessions (100+ rounds). The rocker switch was awkward to use and tended to compromise the firing grip slightly. It was impossible to use one-handed unless switched to the "constant on" position. I found it totally unsuitable for serious social work. This light never left the square range.
The P116D was a good light but a little bulky. I prefer the "disable" option of the P116D to the "constant on" option of the P116C. The pressure switch worked well but was slightly difficult to operate. I had to apply pressure with the tip of the middle finger of my shooting hand in order to actuate it. Simply tightening my grip generally was insufficient.
The P116D-DG corrected the switching issues with the P116D, but not the issue of bulkiness.
I have also used the Surefire 610R, though not on the Beretta. It was a good light and while somewhat bulky, I did not find it as objectionable as the Nitrolon series. I did have the same switching issues. I also did not like the "constant on" rocker switch.
Currently, I am using an X200 with the DG switch. While I am still adjusting to the color of the light generated by the LED's, it is sufficiently bright, not bulky in the least and has the DG switch I prefer. It is a good light and is my preference.
You will note that my preference is for Surefire products and they are expensive. Here is my $.02 on tactical lights: You get what you pay for. I have a light mounted on my rifle, another on my pistol and two (or more) handhelds as part of my kit. All Surefire. Some acquired at taxpayer expense, some at my own. My kit gets abused on a regular basis and even the best equipment breaks occasionally. While I have had Surefire's lights fail me in the field, I have seen far more of the competitors' products go down from less abuse at the most inopportune times. The extra $50-100 a Surefire costs buys a lot more piece of mind.
Further reading:
http://www.surefire.com/maxexp/main/co_disp/displ/carfnbr/91/sesent/00
