I prefer to cut out sheaths using only a very sharp, thin bladed utility knife. If kept well sharpened, this will slide through even heavy leather with little effort. My reasoning is that the least effort will likely help to yield the best accuracy in cutting, which, in turn, will save time and effort later.
A pair of aviation shears, if kept very sharp, will also work for cutting out heavy leather. I have a pair that is used only for this purpose, but I generally prefer the knife as it cuts more accurately and leaves a cleaner edge.