This is a very complicated question whose exact answer depends entirely on which dc-dc converter you are talking about. So, let me first say that modern dc-dc converters will give better efficiency than an LDO down to very low load currents (< 1 mA). Let me also give an example.
For your power specs, the TPS62237 would be an ideal dc-dc converter. At a 100uA load, it would have about 60% efficiency. At load currents higher than this, it has better efficiency.
An ideal LDO would have 66% efficiency at any load, ignoring its ground or Iq current. But at very low loads, like 100uA, the ground current becomes significant and the efficiency reduces further. So, it seems that a dc-dc converter is almost always more efficient than an LDO.
The output noise of a dc-dc converter is entirely dependent on which converter you are talking about. The TPS62237 uses DCS-Control, which is explained in more detail here: http://www.ti.com/lit/an/slyt531/slyt531.pdf DCS-Control is a very low noise switching topology.