Amumu and Hecarim are both good, but they work best with specific types of team comps. I pick a jungler based on what everyone else picked.
If we have no dedicated tank, I take amumu and build straight tank, with maybe a zhonya;s for better initiations and longer up time.
If we have say a tank support or solo top (just 1 tank), I might take hecarim. If we have a tank/beefy support (2 tanks or 1 tank 1 ot), I might take noct, vi, yi, etc (but this almost never happens in normal blind pick at my level).