I agree with Szuszak.
Indeed, paladin healing & fear ward help a lot, and blessing too (salvation is your best friend at Onyxia & BWL).
AQ40 easier for horde, hello poison cleaning totem ! But not so much.
For Naxxramas, i think it's little easier as alliance than horde, BoK & BoW are pretty usefull on fight where survival is the key like Sapphi & KT (but tbh i have never done the 4HM, Sapphi & KT as horde).
But the more important point is to play what you like more