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.
