EFFICIENCY

You should find out what the lowest and nominal impedance is for each of these speakers and that your amp is stable at this minimum impedance. Typically good quality amps are at least 2 Ohms stable. If your speaker dips below the amps impedance limit it will clip i.e overload the amp.

Speaker manufacturers usually suggest the power an amplifier should have to drive their speakers properly e.g 30-150 W @ 8 Ohms. Lets say both speakers have the same nominal impedance of 8 Ohms and minimum recommended power is 30W. This means that the 86db Speaker will need about 70W while the 90db speaker will only need 30W to produce the same volume.

Given your amp is limited to 100W the difference in efficiency can become significant as you would want your amp to run in effortless mode rather than strain as it gets closer to its limit.

Both speakers are 4ohm Thiels,I believe they go down to 2.3 ohm.

The Classe should be stout enough in the power suppply to be able to handle that 2.3 Ohm minimum. The good thing is that this a dip that is not present for most of the musical content. I am guessing one of these spekaers is the CS6. What I can tell you is that watts per channel do matter for these speakers. I had a Krell 300cx for a day before I traded it up to a 400cx and the additional WPC gave me more detail and soundstage with the CS6s. But I also had a large room to fill with vaulted celings. If your room is not too large this shouldn't be a problem for you.

All being the same, I would go for the less efficient speaker. Bigger amps are normally not the quietest animals out there, and the more efficient the speaker the more likely you are to hear noise (if there is any).

Power-wise, it is unlikely that your amp will run out of steam with either speaker.

Regards
Paul

How important is this difference in the final sound quality?

Not as important as how they sound to you. Get the one which sounds better.

The impedance of 2.3 Ohms is a wee bit on the low side....this would concern me as it makes these speakers in the "harder to drive" category. Remember that this speaker will take nearly FOUR times the current of an 8 ohm load and this will definitely restrict your headroom (the dynamics your amp can handle where the amp remains below clipping). A massive amp and the more efficient speaker may help in this case.