Sensitivity is actually not important unless you are powering it with a weak amplifier or direct from the headunit. Just because a speaker is less than 90 (which is almost every high power sub), doesn't mean it's a bad speaker. It also has nothing to do with the sound quality.
It highly depends on how much power you are going to be giving the speakers. Of course if it has a higher sensitivity to start with, it will be louder at first, but if that speaker is only good for 75 watts, and you are comparing it to another at 200 watts with less sensitivity. The lower sensitivity can be louder.
SPL sensitivity efficiency reading is measured at 1 meter from the speaker being powered by 1 watt. 1watt/1meter.
Speaker efficiency 1w/1m (base/start-off) 2w 4w 8w 16w 32w 64w 128w 256w 512w 1024w 89 SPL 92 SPL 95 SPL 98 SPL 101 SPL 104 SPL 107 SPL 110 SPL 113 SPL 116 SPL 119 SPL 91 SPL 94 SPL 97 SPL 100 SPL 103 SPL 106 SPL 109 SPL 112 SPL 115 SPL 118 SPL 121 SPL
Can you tell the difference between 89 SPL and 91 SPL? No human can. 80 SPL to 90,100, yea you'll tell it there, big difference.
Higher sensitivity specs doesn't meant the speaker is louder, it means that the speaker is X dB louder at the same wattage power than the comparable one. Most are going to be around 89-94 dB for mid/high/full range speakers. Subs are normally 87-92 dB.
So you have to compare apples to apples, oranges to oranges. If you have a 91 SPL efficient speaker powered by a 22w RMS head unit, against an 89 SPL efficient speaker powered by a 75w RMS amp, then the 89 SPL speaker is louder. It's not a straight up side by side comparison. It depends on what the max RMS of the speaker is, and what you're going to be delivering to it.
If you're going with powering the mids/full range by the head unit, then yes you'll want the best efficiency you can find, because you're not working with a lot of power there. Powered by an amp, depends on what power you're talking about.