xswanted: I have a Zeiss 6.5-20x 50mm, along with many Leupold's & Nightforce(s). Optical clarity & brightness on my Zeiss is outstanding. Side focus works fine. Clicks are positive.
Depending on the size of your targets at 600, 20x and even 25x may not be enough to see your target in detail, especially when mirage is an issue.
To me, a variable has a big advantage over a high magnification fixed power for finding your target downrange. My standard procedure is to crank them down, find the target, line up, then increase to the highest usable power at that time.
Trying to do the same with my Leupold Comp. Series 40x's can sometimes be a problem when your target is one of many at the longer distances, different distances, etc. For that reason I limit the fixed powers to 300 yd/ mtrs. maximum. Nothing but variables beyond that.
If you get the Zeiss, watch the direction of travel on the turrets. They are the exact opposite of Leupolds, Nightforce, Burris, and the vast majority of others. I got caught a few times moving them in the wrong direction. Overall though, a very high quality scope.