Which is more efficient and requires less power?

Looking at this from the other end, how loudly do you like to listen to music? How large is your room? These answers may have more impact than going strictly by speaker specs.

I auditioned a pair of huge JM Lab Electra 946 with a 3.5W ASL 2A3 amp. It worked great as long as you didn't turn the volume up too much. As an owner of a McIntosh amp with power meters, I can tell you that most of the time, I use less than 5W of power. Maybe you are the same way?

I sit 8 feet from the loudspeakers in a 19x16x8 foot room. I listen
around 89-90db, and occasionally crank to ~101db. Classical to jazz to
heavy metal. At the loudest volume, I'm using around 32 watts. My
present speakers are the 89db/1w/1m in the example above.

It seems 91db, 8 ohm speakers would be a more significant change from
what I presently own in terms of efficiency and driving ease than would
94db, 4 ohm speakers.

Depends on the amp also.
Most solid state amps will double power output when going from 8 ohms to 4. While most tube amps won't increase output ratings at all.
Try this chart also:
@8 ohms for 89db speaker
1 watt = 89db output
2 watts = 92db
4 watts = 95db
8 watts = 98db
16 watts = 101db
and so on.....
every increase of 10db will give you a perceived doulbing of volume.
Compare your amp rating for 4, 6 & 8ohms if you can.
This will let you know what the actual output will be.

Regardless of the speaker nominal impedance, impedance curves, or phase angles, what you will need at your listening position of 8 feet for an SPL level of 89 db is 90 watts for the 94 db/W-M speakers and 285 watts for the 89 db/W-M speakers. This is assuming 15 db peak headroom which is reasonable for classical and jazz.

But which speaker requires more power? Can't tell. If the 89 db speaker stays flat, then all it needs is 285 WPC. But if the 94 dB speaker drops down to 1-ohm at some frequencies, then it may need 360 WPC or more to prevent clipping while delivering the same SPL as the 89 db speaker.

Nominal ratings are a guide, but minimum and maximum power requirements are spec'd out by the manufacturer for these reasons.

Tvad wrote:

"It seems 91db, 8 ohm speakers would be a more significant change from what I presently own in terms of efficiency and driving ease than would 94db, 4 ohm speakers."

Efficiency no; driving ease probably.

I assume you're quoting the 1-watt efficiency figures instead of "sensitivity" figures (sensitivity is referenced to 2.83 volts input; 2.83 volts into 8 ohms = 1 watt but 2.83 volts into 4 ohms = 2 watts. Sometimes manufacturers give the 2.83 volt sensitivity and call it "efficiency". Read closely to see if this is the case, especially with speakers whose nominal impedance is below 8 ohms). Assuming your numbers are efficiency and not sensitivity, then the 94 dB/watt, 4 ohm speaker would be 3 dB more efficient than the 91 dB/watt, 8 ohm speaker.

Now if the 4 ohm speaker manufacturer is actually quoting the 2.83 volt sensitivity instead of the 1 watt efficiency, then its actual 1 watt efficiency is only 91 dB, in which case both speakers have the same efficiency.

In either case, the 8 ohm speaker will most likely be the easier load. I say "most likely" because I sell a supposedly "8 ohm" speaker that's a more difficult load than most 4 ohm speakers due to the unusual nature of the load (it's a fullrange electrostat).

Now assuming that your tube amp is a transformer coupled push-pull type, as long as the manufacturer doesn't caution against 4 ohm loads you probably wouldn't have any problems. I would think that at a given SPL your amp would be less taxed by the 94 dB, 4 ohm speaker than by your present 89 dB, 6 ohm speaker. But I'm not sure about the comparison between the 94 dB, 4 ohm speaker and the 91 dB, 8 ohm speaker.

As you can see there are so many exceptions and caveats and what-if's that a generic answer probably won't suffice (see post by Gs5556 above as an example of how far off my generic answer could be). We might be better able to give useful information if you let us know the specific speakers and amp.

Duke