How to read speaker sensitivity rating?

Neither, the modulus of impedance vs. phase angle determines ease of drive, sensitivity has nothing at all to do with it.

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Okay, assuming the nominal impedance for both speakers is 4 ohms. Does that mean the one rated at 78 db will play louder than the one rated at 102 db at the same power?

I've seen a few discussions here about the difficulty of some speakers to be driven and I often see sensitivity numbers thrown around.
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Again, other way around. The units on the ratings are db/watt/m. So the 78 dB speaker produces an spl of 78 dB, measured 1 meter from the speaker, when driven with an input of 1 watt. The other would give an spl of 102 dB with the same one watt of input power, measured at the same 1 m distance.

The 102db speaker will play louder, and as a rough, perceptual estimate, every 10db increase in loudness makes a speaker seem like it is playing twice as loud.

Mitch4t.
Are you just pulling our legs ?
After looking at your systems I find it hard to believe you wouldn't know this.
Well over 100 discussions.