The hint about schedule 80 pvc is a good one - I was unaware of it. I'm meeting with one of the HVAC contractors hopefully today and I can ask about this.
Given that there really seem to be two camps (sidewall versus vertical) I'm starting to lean towards sidewall because it's the only option that I can participate in mitigation (i.e., ice removal, etc) if problems develop. I have to imagine that 99.9% of the time there will be no problems with either approach as long as it is properly installed. So I'm trying to resist the urge to over think this issue lest I freeze up unable to make a decision. :-)