Which is more efficient and requires less power?

Assuming the numbers given are accurate, the 94 dB-watt 4 ohm nominal speaker is 5 dB more efficient than the 89 dB-watt 6 ohm speaker. In this case, the 4 ohm speaker will also have a higher 2.83 volt sensitivity, at 97 dB for a 2.83 volt input vs roughly 90.5 dB for a 2.83 volt input for the 6 ohm nominal speaker.

I have heard that the convention is to quote a "nominal impedance" value roughly 1.25 times the minimum impedance, rounded to the nearest ohm. There's no guarantee that a given manufacturer follows this convention, though.

Note that a few amplifiers might still be happier with the lower efficiency 6 ohm speaker.

Duke

Thanks guys. I suspected the answer would not be easy.

Duke notes

the convention is to quote a "nominal impedance" value roughly 1.25 times the minimum impedance

Many manufacturers quote the impedance @ 1kHz instead. Not so illuminating (but things are getting better).
On topic -- No1 spkr of course, as all above note.

Duke's qualifier is interesting:

Note that a few amplifiers might still be happier
with the lower efficiency 6 ohm speake.

This is telling, and why I posted the question. The 94db speaker is the
obvious choice, but the more difficult load of the 94db speakers kaes
the answer not so cut and dried, IMO.

The 94 db/w speaker is obviously more efficient, even when both are converted to a common basis for comparison (2.83 volt sensitivity measurement). For most solid state amps, it would be much easier to drive to high volume; the 4 ohm nominal impedance would not be a problem. But, with a tube amp, it is harder to say which would work better. Single ended triodes, in particular, tend to have higher output impedances which translates to less ability control the movement of the cone.

As a pure guess, provided that the 94 db speaker does not have a severe drop below 4 ohms, particularly at very energetic bass frequencies, and if the phase angle is not severe at a frequency where the impedance dips, the 94 db speaker would be easier to drive for most tube amps.