I need a 5V and 1A supply for my portable device. I am thinking of using 5 * 1.2 V NiMH batteries.

Which regulator shall I use: Low dropout regulator or DC-DC converter ?

 

Depends on your design requirements. The LDO is way simpler and less noisy, but the buck switcher might be a little more efficient.... not a huge efficiency gain because when VIN and VOUT are close, bucks are not really efficient. The switch is on most of the time anyway. I would probably use the LDO.

 

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If you use a LDO, your efficiency is going to be in the 83% range. That may or may not be good enough for your application. Also pointed out by bountyhunter is the fact that LDO's are much simpler (usually only a couple of capacitors), while a switcher is going to involve several more components (typically an inductance, a few capacitors, some resistors, maybe some diodes). And there's no guarantee that your efficiency is going to be much better with a switcher.

I'd go with the LDO.