Which plays louder?
...a 88dB/2.83V/M 4ohm speaker. Or a 88dB/1W/M 6ohm speaker.
Here's the math.
2.83 volts into a 4 ohm speaker is 2 watts of power. So it takes 2 watts to get to 86 dB. At 1 watt the loudness would be 83 dB.
At 1 watt the 6 ohm speaker you note is more efficient.
However, there are a lot of caveats that need to be considered.
First of all, many speaker manufacturers are rather "imaginative" when it comes to specs. Even when they are relatively honest, variances in test protocol can make direct comparison of the numbers difficult.
Second, the 4 and 6 ohm impedance numbers you noted are "nominal" figures. Speaker impedances vary quite a bit with the frequency involved. Even speakers considered to have a well behaved, benign impedance will vary quite a bit, especially at crossover frequencies and in the bass.
This trait combines with the fact that no speaker is completely flat across the frequency spectrum. Human hearing is also not flat and tends to be more sensitive in the midrange. As such, one can sometimes get deceptive results with a less efficient speaker appearing to sound louder than a more efficient one.
Finally, amplifiers vary in performance with low impedance speakers. Some solid state amps will double in power with a 4 ohm load compared to a 8 ohm one. Other solid state amps may actually misbehave at low impedance and sound worse. Tube amps tend to deliver the same power output at difference impedances but are dependent on output transformer design.
The important thing is that you listen to each speaker with the type of music you enjoy, played the way you want to play it. If you normally listen at low to moderate volume and your room isn't overly large, efficiency may not be a very important consideration for you. If you are a rocker who likes high volume, you'll probably find that neither speaker you mentioned above will be satisfactory.
Good luck with your audition.
