Both have their own weaknesses. KV-8 is better at AI duty but has a terrible gun for AT duty, Brummbar can at least penetrate Allied mediums and deal good damage if it hits. KV-8 can deal more damage against anything with flesh in its range but it has no Bunker Busting Barrage to bomb things from afar so it needs to close in and expose itself against Panther/Tiger. And KV-8 can't escape easily either since it is really slow. There is also no big price difference, is there? Brummbar costs 150F and KV-8 costs 145F IIRC.
I don't see any reason why either of these tanks should change.
