I think you have a misconception about LDO.
LDO means low-drop-out or when you need a very few difference from Vin to Vout. What you're trying to do doesn't require a LDO, a regular 7805, LM317 or another crap will perform the same (read poor).
You can think about linear regulator's efficiency as Vout/Vin so in your example, it's clearly that 3.3/5 = 66% is a poor number. This means that at anytime, your regulator will heat atmosphere with the rest of 34%.
Even with so poor efficiency, a linear can work very well as long as power dissipated on it (that is, make the difference Pin and Pout) will be adequate for regulator's package + natural cooling or PCB plane (read rising package temp at 50 deg for example). This can be easily calculated from datasheets.
But if you're trying to convert 3 from 3.3 will achieve 90.9%, much better (and cheaper) than most of buck regulators. In this case you will need a LDO (and a good one), since 300mV can't be handled by LM317.
So in your case, buck will be far better in terms of efficiency.