I used a Bushnell 4200 3-9x for quite a while on my 6.8. I thought it was a good mag range for it. I got a good deal on a Burris Euro 1.5-6x40 that is on it now. I like it a little better just because it has the lower bottom end. The 6.8 is really not a long range round, 300 is pushing it for hunting IMO. So something along the lines of 6x to 9x would be about perfect for it IMO.
As far as that scope goes. Maybe I am wrong on all this, but since it is lighter it would make sense that is is not built as robust. Semi autos are significantly harder on scopes that bolt gun because they have recoil going both ways back when the round fires, back again when the bolt goes back, and then forward recoil as the bolt slams forward. Now AR recoil is not to bad, but a 6.8 has significantly more recoil than the .223 version. I have broke several cheap scopes on .223 ARs. Not saying the Leupold is a cheap scope, just saying I broke several. So I would not base your whole choice an light weight in this case, if it comes at the cost of durability. It may come to back to get you in the end on a semi auto.