Neither because lasers work both ways. Also they will only be right on target at one distance. Most are fairly cheaply constructed unless you get a military grade Infrared laser which would usually run closer to a grand and require you to use night vision to see it.
Lasers are useful tertiary targeting aids in close quarters after red dot optics and iron sights for less than optimal shooting positions (you know, like in real life). I have not found lasers to be faster than red dot optics in practice, but everyone is different.
Due to the light wavelength sensitivity of a "normal" human eye, which is highest for "green" light, green lasers are the best for human eyes to distinguish from normal backgrounds.
I use a red pulsed LaserMax laser on my Glock as a back-up to the iron sights. It is primarily for use at night in conjunction with a white light.
The type of lasers that are suitable for use on carbines in life-or-death situations are expensive and you do have to practice and train with them to be proficient in their use. Zero your laser at the distance you are most likely to use it at.
Bear in mind that the path of the beam to the target, even under good atmospheric conditions, is not the same as the path of the bullet and under poor atmospheric conditions (smoke, high humidity, precipitation) the laser is useless.
I like having a laser on my handgun, but I would not put one on my carbine. I purchased a SureFire module for a carbine/rifle setup awhile back and determined that while it was fun to play with I did not want the extra weight on my AR.
High power military IR lasers are for target marking for air support. I'm not sure what a civilian application for IIIa/IIIb IR lasers would be and I'm not fond of lasers that can permanently damage your vision that you can't see with the naked eye.