High vs. Low Efficiency Speakers

The benefit to higher efficiency speakers is that less amplifier power is needed to achieve a given loudness. The benefit of a low efficiency speaker is that you may be able to operate the volume control higher in its range where it has less effect on the sound quality.

Some speakers are grossly inefficient but sound amazing (like Maggies or some Thiels). Those are cases where even low volumes require a minimum of 10-50 watts just to "get them going". Personally, I'd want hundreds of watts with those two but, on the other hand, I'd have a hard time adjusting the volume on a speaker rated at 102db like coincidents or klipshorns.
Find either an amp or speaker you really want, then match it with what you need.

Assuming both speakers have the same size and type of box, the lower efficiency speaker will have deeper bass. High efficiency and deep bass at the same time requires a large box (or a creative marketing department!).

Now if box size isn't an issue, the advantage offered by the high efficiency speaker is typically better dynamic contrast and a greater sense of liveliness. High quality high efficiency speakers often sound better at low volume levels and often have a more tube-friendly impedance curve (apart from needing less power, which in and of itself is a tube-friendly characteristic) - but there are exceptions.

There are other generalizations that could be made, but on the other hand there are exceptions to just about every rule (except for the tradeoff relationship between bass extension, box size and efficiency).

Duke

Duke above did a great job of explaining the factors of a speaker of lower effiency vs. one with a higher one. One other factor to consider is the nature of the load, i.e. the impedance vs. frequency curve. If you have a relatively high voltage sensitivity but a difficult load (one with low impedance and/or high phase angles), an amplifier with a high output impedance could have as much trouble driving that speaker as it could driving a speaker with a lower voltage sensitivity and a higher impedance load. Also, in some cases, your amp will drive the load to satisfying sound levels but its interaction with the load can significantly alter the tonal balance of the speaker.

With your preference for classical music, I would recommend going the higher effeciency route and since you don't listen that loud, try a SET amp out sometime. Good luck.