Theoretically, EVERYTHING can cause some degree of signal quality loss. However, the higher the signal and the better quality the gear (in terms of things like wire, soldering, switches and connectors), the less chance there is of there being any audible degree of signal loss. Better-designed selectors will have circuitry that minimizes or offsets any potential signal distortion or loss, and should have little-to-no discernible effect. A selector interposed in a line carrying a weak signal, such as that from a turntable, could have a big impact; one selecting between speakers (e.g., passing a high level signal) would have much less impact on the sound.
All that said, a purist will always seek to have minimal potential disruption to the signal, and won't use a selector unless the convenience of using one outweighs the potential signal degradation.
The Adcom selector is quite good; I have no experience with the Niles, but the brand has a good reputation. Entech (I think that is the correct spelling-?) also makes some good ones, in heavy professional cases.
You might also consider system selectors, which allow selection of not only different speakers, but also different sources and amplifiers. They can be very handy if you have a large collection of gear that you want to keep in regular use, or try in different combinations to seek synergies. Of course, using them means using a lot more cable, which can get expensive. If you have a typical vintage system, there are vintage-era silver-fronted ones from Sony, Pioneer, and other brands that blend well cosmetically. Be prepared for cleaning and re-soldering, though, and perhaps even re-wiring, to make them transparent enough for a revealing system. Most of them have rhodium-plated connectors, whereas the newer hi-fi ones like the Adcom have gold plating over rhodium, as gold doesn't tarnish.
Whatever brand you try, make sure to periodically clean the contacts well with DeOxit and ProGold, to ensure the signal passes through the connectors with minimal degradation.
If you can arrange it, I'd suggest A/B testing the sound with and without the speaker selector in line, to see if you find any detectable loss of sonic quality. In a really revealing system, the difference may be audible, but with a good-quality selector like the Adcom, in most systems it usually won't be.
That's a long-winded way of not really answering your question (sorry!)... maybe someone else will have A/B'd the Adcom and the Niles, and can give a better answer.
EDIT: Looks like RT Fan types faster than I do...!
