In my honest opinion.

Questing wins.

For two reasons.

Reason one is that you can get kick arse xp if you hit it hard over a few sessions. look up your quests on say wowhead, or use addons such as questhelper so you dont waste time. Big chains are big...and juicy xp runs that are usually easy as all hell.

Reason two is that you will make an absolute crapload of cash without even trying. It adds up crazy fast. If you are dirt poor and need to gear your crew in crafted items as soon as you hit 80 then this is the way.

Of course, @ 80 you can run through the same zones if you dont clear them. And make a crazy amount. If questing is your thing.

I found instancing somewhat slower, and became very repetitive quite quickly.

I quested my shaman from 72-80 without touching an instance. Very, very quickly.
My lock crew are almost at 80 and ran from 75-78 in about two evenings of effort. Plus each made 1k gold in that time just off quest rewards. And that was just doing the 100 or so quests in Dragonblight.