To the OP:
I have been competing in F-Class for a number of years now, and i can tell you that the ovewhelming majority of F-Class shooter use high-magnification variable power scopes. As PetePan mentioned the Nightforce 12-42X56 BR scope is the paradigm in F-Class. Scopes such as Sightron, Leupold and March also have followings. The reticle choice and the adjustments (1/8 MOA vs. 1/4) is what influences scope choice, and it is one reason why Bushnell and Zeiss are not popular choices in spite of having pretty good scopes otherwise.
Fixed power scopes will work, in fact I have a Weaver T-36, which is a fixed-power 36X scope. It is a gem with 1/8 moa adjustments and a 1/8 moa target dot. Unfortunately in sunny conditions at long range, when there is a great deal of mirage, having that much power is a problem. All you see is a wobbling fuzzy blob.
The ability to dial back on power is pretty much essential for best results. In Winnipeg last summer, I was shooting at 22X on my 42X scope in some relays. Chilliwack and Calgary were exactly the same.
One will never regret investing in good quality glass. It will last you a lifetime, and you can port them from gun to gun with the advent of quick release scope mounts such as the Leupold QRW's.
I often say it takes a wealthy man to buy cheap scopes, because in the end, those folks spend way more than if they just did it right in the beginning.
Don't take my word for it though... come out to an F-Class shoot and see what guys are using... LOOK through them.... COMPARE what is out there. They will explain graphically why so many free suggestions on the internet are out to lunch.
Ian