It just depends on how much stamina you have. If you have tons, then there is no reason not to play on hard. If you're low on stamina just play on normal otherwise you'll get tired quickly and make mistakes.
This is one of the main reasons why running a duo at any position (even QB) is awesome. Less stamina needed to play the whole game at Hard intensity, and you can dump the extra points into throwing, vision, strength, SAs, whatever.