Both are good, depending on your particular fighting style.
For an offensive style, a punch lets you be more mobile and aggressive. That Glaive/Sword about to break your shield? Throw it at him and use that split second distraction to take out his legs or go for a kill shot.
A Strap shield, though more defensive, can actually be very aggressive for shield-to-shield fighting, such as checking and rushing.
For someone who has never fought before, I would recommend a punch shield, as it has an easy learning curve. See a shot coming towards your right side? Put the shield there. Going for your leg? lower it slightly while pulling your leg back.
A strap shield requires more body mechanics, as you'll lean your body towards the incoming shot to block it, since you can only move that strap so far.
I actually like both types of shields; a punch for close quarter/flanking, while a strapped for defensive line battles. A small strap shield on the upper arm allows more freedom, such as while doing archery, 2-handed Flo/Weapon or spear, while the backpack strap also provides protection with some mobility.