Parallel preferred, because speakers are driven by the amplifier, not each other. Another way to put it.... No two speakers are exactly the same or will react exactly the same, so when they are in series, they affect each other. When in parallel, and assuming adequately low impedance wiring, they are driven tightly by the amp. Of course, a higher damping factor will mean more control over the speaker, and it matters even more when the speakers are in parallel. YMMV.
Cool topic!
-Tommy O