Thanks for all the posts and information so far.
The feedback on the SS 3-9 was helpful, since that is a good zoom range, and overall durability is just as, or more, important to me than a higher magnification or better glass (given a $600 or so budget). I am looking to set this up on an AR in the category of a "designated marksman" field use role, not a bench rifle, and not looking for minimal groups on paper at 500 yards either.
The Vortex 2.5-10 looks like a good option as well. A good feature it has is the capped turrets instead of the exposed target turrets, since for a field rifle at moderate distances with an AR, there is not too much clicking needed.
I asked this question because I'm under the impression that for a given price, the fixed power scope is likely to be more durable, but the recent high quality variables have closed the gap significantly. The 6x SS is a nice scope, and has a great reputation, but that 6x is tough to use at closer ranges in the woods.
I'll add that I live in the north east, and given the terrain around here, even 400 yard shots are not the norm. I used to live in Arizona, where it was flat and dry, and you could see targets at so much further, it was a shock to me. Hardly so in the woods and hills of Pennsylvania. I would say that for my use, 2x per 100 yards is a good balance of being able to resolve targets versus field of view. I like higher magnification at any range when I have a stable rest and a fixed target, but the lower magnification works better in field conditions.
As the 1-6x scopes continue to get better and become more affordable, they would be ideal, although the other scopes have the advantage of the larger objective lens. A 1-6x40 would be nice for me, but then again, everyone has their "ideal" preference.