Depends on deployment and if I was using my good list or my fun list list. If I qas using the good list then probably the battlecannon just because it has the range necessary to hit the mobile section of my army no matter what it does, the demolisher I can move around. With my fun list I'd be worried about the demolisher cracking open my AV14 tanks which add much needed muscle.Ask yourself this if you were in a mirror match with IG would you be more afraid of the battlecannon or demolisher?
Yeah but we're IG. What about Marines with their double land raider lists? Or any one with AV13 vehicles? Who are you more scared of then, then guy who's on 5's/6's to glance on 2D6 and pick the highest or 3's/4's on 2D6 and pick the highest?
You can pretend if you want to and say demolisher, but we all know that you would hate to see that battlecannon shooting at your manticore or chimeras every turn.
How about infantry? If you have a lot of 2+ saves in your army, what are you more scared of? The Demolisher at AP2 or the Battlecannon at AP3?