Not to forget that if you are perking into the agility tree, it boosts the damage of the SMG a lot, both through Agility itself and Gunslinger. Throw in Hidden strike and the SMG is going to be the better weapon if you max out those perks compared to an unperked M60
So:
Maxed out Agility - Double damage for headshots
Maxed out Gunslinger - 50% increase in damage
Maxed out Hidden strike - 300% increase in damage for hidden strikes
So comparing Q1, Average stats between a perked out Agility and a perked out Fortitude player
SMG - 225 headshot sneak damage with regular ball ammo
M60 - 140 headshot damage or 175 (if they take the first perk of hidden strike that they can)
Throw in that both classes has a book series that gives them both sweet benefits, along with the Gunslinger perk having the gunslinger combo bonus in addition.
If you are playing a true stealth player, then you will be perking into Agility to boost sneak damage and the SMG wins hands down. If you are not perking into stealth, then any weapon with higher damage output is going to be best regardless of sound level.