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Efficiency of a linear regulator depends on its output voltage and on the regulation of its input voltage. Suppose the output voltage is 5V and the input voltage varies ±15% plus an additional 0,5 ripple voltage. If the drop voltage is, say, 0,5V, the minimum input voltage should be 5+0.5V. Taking into account the 15% tolerance and ripple it should be increased to 6.9V average. So, the efficiency would be 5/6.9= 72% average. This is an example only and usually you should not achieve such a good result.
By the other hand, the efficiency of a SMPS depends only on its topoly, design and component quality. For low voltages the SMPS will always win.
The key to take a decision is: how is the regulation of the input voltage?
Added after 1 minutes:
Obviously, the LDO will have the best regulation, but a well designed SMPS can rival it.