Originally Posted by OpticsCamp

Argon vs nitrogen doesn't matter. They both achieve the same goal of purging air and moisture from the body, preventing fogging. The only real difference between those two gases (when used in scopes) is their atomic weight, meaning an argon-purged scope is going to be about .0000000000000000001 oz heavier.

Objective size - I don't think you'll notice any difference in light transmission between those two sizes. Go for the one that will fit lower on your gun without hitting the barrel or any other part of the gun.

Eye relief of .3 inches might be slightly noticeable if you're picky, but it's pretty standard. The longer eye relief of the Monarch gives you a little more room to find a comfortable mounting position.

Whether or not the $40 is worth it is up to you. I'm fairly certain that both Vortex and Nikon at that price range are manufactured in Asia. Vortex is a little more clear on their product warranty, but either one is a fine choice.