Which spectto level through Warlords of Draenor regarding looms
My main is holy and offspec is ret, but I have a holy heirloom mainhand weapon that will carry me to 100.
Now do I stick with holy because of my level changing weapon, or go with ret for max deeps?
If you want to level slowly go Holy (since I assume grinding dungeons won't reward the best XP). I would recommend going ret, the heirloom weapon is nice but you'll find stuff that's equivalent I bet.
I believe heirlooms are equivalent to a blue item of your level (roughly).
That being said, whatever spec you level as is really up to you, since you have to enjoy it enough to get up to 100.