Questing will be the most efficient way otherwise you run the risk of becoming cash strapped, especially since slicing is no longer very profitable via missions.

I leveled up my commando's through a combination of grinding and questing and the grinding was only marginally faster than questing but I made no money. Ultimately grinding ended up costing me nearly an entire day of leveling time as I back tracked to catch up on my story so I could finish out 50 on Ilum. Still managed server first though ;p

I'm doing two sentinels now, money is not an issue. They just hit level 35 and I started them the night before last. They are 100% pure grind mode with decked out ship droids to heal.