Let's say that you've got a 4ohm 88db speaker (okay in my case it actually dips briefly to 2.5ohms), and a nominal 8ohm 84db speaker. Provided you have amps that can power both, would there be a reason to pick one over the other? Would the higher sensitivity of the 4ohm sound more dynamic than the 8ohm? Would the 8ohm conversely sound better due to pulling less current?
Depending on whether sensitivity is based on voltage rather than power the difference in sensitivity could be as little as 1dB.
There is no reason I can see to pick one over the other based on sensitivity, a better test would be to listen to each with the same amplifier and determine how they sound different. Speaker choice is a matter of personal preference, note that level matching is extremely critical, the louder sounding speaker at least in the short term is going to be preferred unless there are grossly obviously deficits in performance of one of the ones being compared. This is despite the fact that one may in fact be subtly preferable to the other in a well controlled comparison.
A variety of casual to rigorous test methods may be employed, just make sure to level match. (Pink noise is good for this)
There is no reason I can see to pick one over the other based on sensitivity, a better test would be to listen to each with the same amplifier and determine how they sound different. Speaker choice is a matter of personal preference, note that level matching is extremely critical, the louder sounding speaker at least in the short term is going to be preferred unless there are grossly obviously deficits in performance of one of the ones being compared. This is despite the fact that one may in fact be subtly preferable to the other in a well controlled comparison.
A variety of casual to rigorous test methods may be employed, just make sure to level match. (Pink noise is good for this)
At what frequency would you level match? Do you EQ them to match each other? Do you actually do this when auditioning speakers? I'm never sitting there flipping back and forth between level matched speakers at the same spot with the same spot in source music nor do I think that is even remotely necessary with the very large difference between speakers. Not to mention you are usually stuck auditioning them in different locations or at different times.
Lets just say that both will have dips and the amplifiers are capable of driving the loads.
Lets change the sensitivity of the 8 ohm version to 85dB so that things are on a par with one another.
Typically the only reason to go to 4 ohm vs 8 is to reduce the amount of voltage swing, that the amplifier has to be capable of, to deliver a specified amount of current. Or said another way to get more power out with voltage rails that cannot be driven any higher. Such as cars, battery operated equipment and pushing the envelope for even higher levels of amplifier power in either a domestic or PA environment.
The advantages of going to 8 ohms are less losses in the cables, less losses within the amplifier itself, higher amplifier performance, as distortion typically decreases with increasing load, lower impact from the systems output noise as the speakers voltage sensitivity is lower and reliability should be improved too - all things being equal.
I probably missed some, but you get the point. There is no good reason to go to 4 ohms unless you have to. Either because of driver limitations (your drivers are only available as 4 ohm) or voltage limitations.
In some circumstances it makes sense, such as with subwoofers and plate amplifiers. Sub drivers tend to be low sensitivity, so noise won't be an issue. They tend not to have good high frequency reproduction so any noise is largely masked by their own natural low pass construction. Distortion degradation, from low ohm loads, doesn't manifest as strongly until you hit higher frequencies. Cable losses are kept to an absolute minimum because the driver is normally located right next to the amplifier. Using lower ohm loads here allows for the use of lower voltage rails in the sub amplifier and as such you get to use parts (caps and semiconductors) with lower voltage ratings. With caps this reduces cost and size and with semiconductors it allows you to either use lower voltage parts, or use more of the devices safe operating area, for more power with fewer devices.
An amplifier/speaker can display Voltage limiting or current starvation. Current limiting can be a sad form of distortion that is not as easy to detect as Voltage limiting. Either is a sign of a bad combination. In the absence of any clear evidence I would tend to err on the side of light current.
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