I have a level 55 Paladin that i've had sitting on the shelf for, well longer than I can remember. and i decided it's finally time to dust him off and take him to the top. So my question is, with 3 Pieces of Heirloom gear ( chest, and shoulders both with 10% more xp gains and a back which has 5% ) would it be smarter to roll thro instances, or to simply stick out and run quests?

Considering the dungeon queue system still takes forever (unless you're a tank) is recommend questing in between wait times. If you're a tank I'd alternate between a period of dungeon grinding and question. Both give good gear and experience. Dungeons seem to give better gear though bit since you're in heirlooms you shouldn't be too worried about gear

Go Ret and rock quests you will sky rocket, they've augmented quest lines and new quest areas to make leveling much faster and Retadins have no downtime when grinding through mobs.