I have to agree that Green is not as ideal for defensive situations because in my experience (I used to be an armed bodyguard) the red lasers were always much more intimidating whereas nowdays, green lasers are still not much more than a novelty. There was an incident going back some years now wherea red laser defused a situation for me ....from what was almost certainly going to have resulted in a some form of lethal exchange since I was severely outnumbered at the time. The red dots intimidated the aggressors and they left immediately.
Green sights are certainly used by US forces in Iraq at the moment but again, this is more as an offensive accessory since it intimidates an easily impressed enemy which has little or no understanding of anything other than an automatic rifle from the AK family. A millitary sniper will use an invisible IR laser and a scope/sight capable of seeing the resulting dot. I would be inclined to get green sights for the pistol range and red sights for any personal use/ The heavy-duty nature of the Reds make them less fragile. You can also get red lasers that are built directly into the recoil lug (below the barrel) for numerous models. These activate when the slide is racked. For revolvers, I know you can mount a laser to them but find them cumbersome compared to automatics.
I bought one of the much brighter <35mW green lasers sights from eBay last month and was pleasantly surprised with it although I am not going to be using it for a weapon sight. It will depend on your needs but if you want a green over the reds, the brighter dot makes your firearm quite accurate in sunlight as you can see the dot much more clearly.
(this is the <35mW laser I found on eBay for US$62 + postage)
Some pics here...
http://www.pbase.com/nero_design/lasers_other
I suspect though that the calibers over 9mm may cause a massive shockwave when firing the weapon and any 9mm round over 147 grains is likewise going to subject the green laser sights to a lot of concussive vibration. This may move the KPT Crystal and diminish the usefulness of the laser although logically, you imagine anyone selling a green laser would make them sturdy enough to actually be used for the purpose for which they were/are sold.
Take a look at your model of firearm and if it's a Sig, a Glock or even a .45 ACP, you may be able to get one of those amazing slide lugs (AKA: 'guide-rod') with the laser built into it. Here's a guy who sells them and I can recommend them myself.
http://www.shooterssupply.com/html/lasermax_internal_guide_rod_la.htm
The number one reason I suggest a built-in laser for a defensive pistol is that 9/10 shooting occur within 10 feet of your opponent. For this reason, you may never need a laser sight except to avert confrontation since many perps will surrender when they see the dot. However, the bolt-on laser sights (like the green aboe which I purchased the other day) are too large and they OFTEN catch on holsters. I wore a <5mW Red laser on a Smith & Wesson 9mm 5904 and found it was always snagging on my pancake (in the pants/low profile) holsters. Both leather and nylon holster too. For this reason, it became a liability and the metal claws and clamps damaged the finish on the firearm as well.
Hope this gives you some thoughts. The colour you get will depend on your uses and needs.