Having been away for awhile I decided to give D3 another shot. I don't have RoS yet, so please keep and answers restricted to pre-expansion.

I am looking to maximize my experience gains and don't know which is faster. Lower difficulty level due to faster runs or higher for better gains? And should I be killing every monster in my path or trying to complete quests as fast as possible?

What I have been doing is running through an area killing any elites along the way and any big groups of mobs, but trying to finish a scenario as fast as possible. I can fight at medium speed on Expert level. The other two levels above that would be very slow at my current equipment level.

Any suggestions?