I don't even consider the rest of the game as part of the game. It's just training. End game PvP and PvE is what the game is about. Since I'm a pally, I've done both tanking and healing for end game raids, and I can definitely tell you which is easier.
As for tanking and healing at the same time? The other day, since my main tank sucked ass, I grew aggro on a boss fight in Kara even though I'm a paladin (pally heals generate low aggro). He actually couldn't get aggro off of me, and I just kept healing myself, and we downed the boss.