:thinking:
hmm...
I believe you may have a misunderstanding of termonology. (EDIT: Sorry, I just read your post wrong

, still here is some brief information! )
mw is the power of the laser. 5mw of green (532nm) is relatively low powered. It will not burn, but it will be very visible to the human eye compared to 5mw of violet (405nm)
nm is wavelength, or in other words the color of the laser. 405 is purple, 445 is blue, 532 is green so on and so fourth. Some colors cost less than others because of their wide use in products.
Back to your question, you will see the beam of a 5mw green (532nm) laser and any power higher. The higher the power of the same wavelength the more visible! (so 100mw of green will be a lot brighter than 5mw of green, but 5mw is still enough to see the beam)
PLEASE NOTE: At high powers of lasers, PLEASE USE EYE PROTECTION! One flash of a laser into your eye could permanently give you a blindspot! 5mw isn't too bad if you accidently glaze over a mirror and get yourself in the eye. But don't stare into the beam on purpose! 100mw of green into your eye will be pretty bad, it will hurt your iris, and possibly permanently damage your retina

.
But, if you DO decide to buy higher powered lasers, it is always nice to have a 5mw green in your collection. Most people have not seen a green laser, and although it will be boring to somebody who has an extensive knowledge of lasers, sometime it's nice to take a break from high powered lasers. Just the other day, I was playing with my first red laser (2mw) and was having some wild fun just pointing it around the house at night!
That being said, higher powers of other colors will more or less visible than others.
However, every human sees colors slightly differently than every other person.
For instance, 50mw of a red laser (650nm) will be LESS VISIBLE than the beam of a 5mw green laser!
For reference: The red lasers many people had in 2000 are 1-2mw of 650nm!
I hope this helped, mike!
EDIT: Although it's not too accurate, you can use this as a reference for brightness of different wavelengths!
Luminous Efficacy