Hi Kevin & Zap,
You don't want to store any heat in the collector itself. The lower its thermal mass the better. Low thermal mass allows it to heat up fast when the sun first comes onto it, and to not have much heat wasted when the sun goes off of it. Any heat you store in the collector just ends up be lost quickly out the collector glazing when the sun goes off the collector.
I'd use the thinner wall copper tubing just because of the cost, but I think that either will work fine.
I don't think that you need to worry too much about special cleaning of the inside of the pipe. When water is flowing through the collector, a thin film of slow moving water forms near the pipe walls, and this film does not conduct heat very well into the main flow of water -- this film of water will be the biggest thermal resistance, and the copper wall will be so much less that it hardly counts.
Gary