I've been very pleased with Vortex as a company and can recommend their products and service to anyone!!
You might consider going a few dollars over and getting into the Vortex Diamondback line. Also, the Nikons are not a bad options by any means.
You'll need some pretty fine crosshairs for shooting at 600 yds.
Edit: Try some of these scopes side by side if you can, then decide
I was thinking what Tony said. I have the Crossfire II on my AR and really like it, but I do all of my shooting inside 300 yards. At 600 yards, the crosshairs would be pretty much covering up the target???