Re: 600 yards- 4.5-14 or 6.5-20?
If you're considering going with Nightforce, you should probably decide what the closest distance is that you will be shooting. When comparin the 3.5-15X to the 5.5-22X, the larger scope is less than half and inch longer and there is only a difference in weight of 1oz. Unless you need the lower bottom end, there really is no reason not to go with the 5.5-22X. The only drawback would be if you never (or rarely) run it at 22X and want to use a ranging reticle.
Since I use an NP-R2 reticle, I just range at 11X and divide by twice the MOA. Or, I can just dial up to 22X, crunch my numbers and dial back down. It's easy to do on a Nightforce since the whole eye piece is the magnification adjustment.