personal choice really. some argue that the cannon is gonna get shot off sooner or later, so having sponson guns keeps the tank useful after the first ''weapon destroyed'' result. there are also times when you won't want to fire the battle cannon, like when the enemy are very close to your own troops (trust me, the 6'' scatter can be very unforgiving if you have, say, you command HQ within 6'' of the target. that wasn't a good day!)
on the other hand, maybe the battle cannon won't get shot off and the enemy will keep their distance, and you end up wasting the points.
for my money i always take heavy bolter sponsons, even on demos, just in case. |