I have used a number of Crimson Trace lasers, both red and green. They have maintained their ability to hold zero, and I have yet to experience a problem with any of them. What I would really like to see Crimson Trace do is to make their XD grip laser in green. I have the LG-445 on my XD45 5" Tactical. I don't have the largest hands in the world, and the shape of the Lasergrips really adds to my enjoyment of the XD.
The best of all worlds for me would be the Lasergrip in green. That being said, a rail mounted laser has a lot going for it - you can move it from gun to gun, and battery changes are simpler that they are with the Lasergrips. One thing you have to take into account with a rail mounted laser on a handgun is the availability of holsters - DeSantis is probably the best at offering holsters for different models of lasers. If this is for your carry gun, you might want to see what holsters are available for your intended combination of laser and handgun before making your decision. We get a lot of people who buy a laser first and then are disappointed if there is no holster to suit them.
DeSantis supports the LG-469 Laserguard (that attach to the trigger guard) for the XDs, Galco shows no holsters for an XD with any laser. Safariland list holster for the XD with rail mounted light /laser combinations, like the Streamlight TLR-2:
I really like the Crimson Trace products - and what they call "instinctive activation". There are a couple of reasons I like it over the Lasermax guide rod lasers - the Lasermax is not adjustable for elevation or windage. Lasermax guarantees 2" at 50 yards, which is more than adequate for self defense distances, but I like to fine tune my lasers for specific distances and loads. The Crimson Trace is adjustable for elevation and windage. It always bugged the crap out of me that I couldn't perfectly center the guide rod laser.
The other reason I like the Crimson Trace grip lasers is the activation. Activating the Lasermax requires pushing a button that replaces the take-down lever. The Crimson Trace Lasergrip activates when you grasp the grip firmly. In a self defense situation, fine motor skills (like pushing s tiny button) deteriorate quickly. Gross motor skills (like grasping your gun) still function.
I've gone a bit far afield of your original question, but I like talking about lasers...
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