Yeah, I agree that, generally speaking, any difference between hi-res vs. redbook 16/44.1 is largely due to the hi-res version being "closer" to the master and/or better mastering. The reason I buy from HDTracks is for the convenience and immediacy of getting the music. Unfortunately, when I purchase the music, it's in a format that my whole home audio system (Sonos) can't deal with!
The Sonos, as you've pointed out, is awesome in terms of it's convenience and ease of use. Man, if it could only play hi-res formats!