its really hard to tell which is best as the companies make spks targeted for masses and so they give very scarce specs. on z2300 u have a pretty good signal to noise ratio which may stand for higher q. components used. in practice or rather theory, at 100db u wont hear any noise /hiss/ coming from speakers themselves.
as for 1 or 2 drivers per satellite there is hardly any comparison possible. look at the s e r i o u s home theater speakers and notice how they all use mid-range and twitters plus a bass driver in the same case. if smb still not convinced do a comparison between car speakers and notice how freq. range changes based on drivers.
so, from what they provide for the price i would prob go for z and later would upgrade satellites to 2 way speakers.

http://kenwood.ca/Dealers/Ce1/flyers/2005_Speakers.pdf