I prefer both.
It depends on the type of IG army I am playing. For a standard infantry based IG army, I prefer a Russ so it can sit back at range and kill enemy troops.
For a Grenadier/Mechanized list, I prefer Demolishers to go up with some Hellhounds and draw fire away from my Chimeras with my beloved Stormies:rofl.
Either way, I equip my tanks with EA, Smoke and the triple Heavy Bolter combo.
I picked the Demolisher, because that is my favorite way to play.:happy:
Cheers, DH