For one I actually know a few who would see both (mostly girls, though). But that's not the point really, it's just that every year Saw has always been number 1 in the Box Office, and now because of HSM, it won't be, they could have easily released it a week later, but also the fact that the theater is probably gonna be crowded because of HSM.
HSM is probably gonna beat Saw not on the basis of it taking away viewers, they still have their respective audiences as well as a handful for both, as funny as that may seem. It's just, Saw V has a limited audience because of the 18+ thing, whereas anyone can go see HSM3.
I mean, it still has a chance of winning that #1 spot, unlikely though.