Cannondale F5 or Specialized

My son is looking at mountain bikes, the F5 and the Specialized
Rockhopper (I think). They're both comparable in price, in fact at
the LBS the Specialized is about $100 less but seems to weigh twice as
much. Can anyone offer any criticism of either bike? I'm strictly a
roadie and know nothing about MTB's. My only advice to my son was
half the weight was worth at least a hundred bucks to me.