That seems to be rather debate able.

Questing is fast as hell... however collection quests can be a major slow down. Personally I purchased Zygors guide, and Im following it with all 5 toons, we simply skip the non chained collections, and it makes things significantly quicker.

Instance running with a 70 can be fast... but you miss out on the exp of learning your team. You might think, meh I have the basics down, then you get jumped by a warlock who fears your main and you realize your macros dont swap to the next toons.

Leveling by questing imo is the best way... you gain levels pretty quick with the new patch, and on top of that you get to tweak your setup as time goes along.

Saving you a hell of a lot of frustration at 70. Instead of spending hundreds of hours tweaking when your finished. You can tweak along the way, and enjoy it at the end.