I disagree with the above statement. If you've spent any credits on the Bonus XP for FP's, while lvling if you kill the majority of the mobs in FP your going to get more XP than running around a planet killing mobs. If you q at a proper lvl you can bang out about 1/3-1/2 of a lvl with just killing mobs. If you do bonuses and have the daily quest and the quest to complete said FP you can gain almost 1 whole lvl in roughly 30-40 mins. I'm not talking going in and killing every single mob in the instance, but if you kill the ones in your path it's well worth the time spent.
Sorry, but I'm calling bs on that. My latest 50, my 47 and 45 where leveled exclusively by doing only dailies (wz and fp ones), class missions and a few extra WZs, and your statement only held true during double xp weekends,, unless we're talking extremely low level, around 15-20 or so, with an xp boost active at all times and rested xp.