DSII - Big guns vs lots of little guns

I've just started playing DSII & I'm wondering which direction is better
for tanks, using multiple guns, or only one big main gun.
One design I've done has a triple DFFG/2 turret (size4), & another has
SLAM/5 turret (size3).
Could anyone help with their experience of the effectiveness of each
philosophy?
Also, do you roll once to hit with multiple mounts, or roll separately
for each weapon?

Brendan R.
Hit'em fast & hit'em hard.  Show no mercy.