If I was looking at a new gun, I'd take the one with the rail. Anytime I'm checking open doors, etc in a building, I always need a free hand to open doors, pull back curtains, whatever. Having the light mounted on the gun means I always have a hand free.
I also firmly believe in target identification, and a laser might show me where the bullet is going, but it doesn't tell me if the target is friend or foe. I want to know what I'm shooting at first. As for the light making you an easy target, if you're carrying a flashlight, while clearing a building, you're already pretty obvious! Unless your light is shining right in the bad guys face, he already knows where you are, and holding the light in the other hand won't help you.
There are times when having the light in the off hand can be good, I think a quick-release mount is a great idea, so the light can be carried on the duty belt or in a pocket until needed.
PJ