I moved into a house with a whole house wired system called Elan, which distributes a sound source to an amplifier and then on to rooms with ceiling speakers, each controlled with a wall mounted volume control. Not being familiar with HTD, I can only presume its system may be similar.
I also have incorporated Sonos into my system, both for programming source options and flexibility, and for rooms not prewired with my Elan. One cool thing is that by buying and using a Sonos Connect (zoneplayer 90), connecting its analog output to my living room's Home Theater AVR, then using the Elan's analog inputs to the AVR and selecting zone 2 (AVR setting on my machine), Sonos sound sources can be sent to the prewired ceiling speakers.
IMHO, the sound from ceiling speakers vs Sonos bookshelf speakers is a different audio experience. Ceiling speakers can be single (mono) or paired (stereo) in rooms and are great for background, ambient listening. Sonos speakers can be near ear levels, and in my case, are a more satisfying music audio quality experience, and can be paired within a single room for a traditional stereo output.
So, generally, perhaps you can decide between the two based on the type of audio listening experience you generally desire, ceiling vs bookshelf. Of course, ceiling speakers take up no space, countertops or tables. Hope this helps you somewhat.