I have a mixed bag so to speak... I have a fixed 4x on my whelen, a fixed 2.5 on my 375, a fixed 4x on my 270... I have variables on all the others. My dad has a 2.5x fixed on his 06 and has killed quite a number of elk streaching out to 300 yards or so with it... and I've seen him smoke yotes out in the 450 yard range with a fixed 6x on his 22-250!
I, like the rest of you generally keep my scopes set in the range of 4-6 depending on how far I think the shots will be etc. etc. I really don't have a prefrence between fixed or varaible, but for a rifle that I would be taking shots at large animals (elk and muleys) at normal ranges (inside 300 yards) I think either a 4 or a 6 would be perfect. I really like to dial them in if I have the time though, especially on longer shots, and especially on small critters like antelope. I've shot quite a number of unsuspecting elk with my scope turned up to 9x, just to make it easier on me to make the longer shots... I purchased a 4-12 for my 280 last year. I shot a deer and an elk last fall with it both set on 6x... The deer was about 90 yards and the elk was at a skosh over 300... IMO, the variabls are just as dependable/rugged as the fixed... If the POI changes between the power settings, send it back!
The weight savings is going be probalby less than 2oz too... I belive the fixed 6x40 that I looked at was about 11.5oz, and the 4-12x40 that I got weighes about 13oz.