Pretty much the same current is required to drive both speakers to the same loudness (speaker Y will be louder by about 1.5dB - not detectable).
high sensitivity + low impedance vs. low sensitivity + high impedance
OK, so here's a hypothetical.
Speaker Y has a rated sensitivity of 90.5 dB and a nominal impedance of 4 ohms (but it actually drops to 3.2 over some of the range).
Speaker Z has a rated sensitivity of 86 dB and a nominal impedance of 8 ohms (but it's actually 10 ohms or more over much of its range).
For a given amplifier, which speaker is harder to drive?
Speaker Y has a rated sensitivity of 90.5 dB and a nominal impedance of 4 ohms (but it actually drops to 3.2 over some of the range).
Speaker Z has a rated sensitivity of 86 dB and a nominal impedance of 8 ohms (but it's actually 10 ohms or more over much of its range).
For a given amplifier, which speaker is harder to drive?