Just personal observations, some logic and basic facts would seem to indicate that heat would kill more. More people venture out during warmer months, and cold immediately makes you want to put on more clothes, and rethink your decision. I would also say that it is more likely that the people who venture out in the cold are generally more experienced, or they would be more likely to stay in. Just guessing.

I have personally seen more people push their limits in the heat than in the cold. I think the two extremes operate differently on the human psychology.