Just finished HoO normal tonight and have done Grim Batol normal also. I would recommend HoO since the bosses seem easier (to me at least) and there are more of them. Each boss gives 150 rep, so that's equivalent to 15 normal trash mobs. 7 bosses and not too much annoying trash makes this the easy choice for me. I found the trash in Grim Batol much harder than HoO.