Take Doug up on his offer. He will hook you up with a great price on a nice optic; and will give you several options. CameraLand sells most of the brands mentioned in this thread so he doesn't have to push one brand over the other. Just lays it out straight and you can decide what makes the most sense. I've seen him jump in to help folks on this forum get their optic serviced, even if they didn't sell it. Very few vendors out there will go to that level.
My .02 cents is If you can deal with the 4x on the bottom end, I think the 4-16 ATACR is a better optic than the mk5 3.6-18 if durability is a primary consideration, but either should meet your requirements. To my eyes I can't tell much of a difference in glass quality between the two.