Mark,
Being an engineer with a thermal background I was intrigued by the premise that bottom freezers were more efficient. After thinking about it a while I actually disagreed. So I went digging to find more authoritative answers. I did find a few references and claims online that stated "According to US Dept of Energy... top freezers use less energy than bottom freezers...". But I did NOT find any such statement directly by DOE themselves. So I question the legitimacy of those vendor claims.

From a strictly engineering perspective of efficiency the most loss is when you open the door. So if you want to reduce your energy consumption by the unit, stop opening it so many times and quickly close it. The bottom/top argument seemed to pale in comparison to the 'teenager' dilemma. (teenager opens door of fridge, stares at the contents for 60 seconds trying to figure out what they want to eat...)

My experience is that the side-by-side is most convenient as it provides the most commonly used items (fridge and freezer side) to be accessible at the most convenient height without having to bend down. But that's just personal opinion.

Of course a bottom freezer unit would likely be cheaper than a top freezer unit. Simply based on market demand, not necessary the real cost to build the unit. But then why not go with a unit that does NOT have frost-free. I bet you can pick those up for a bargain these days. On second thought.. bad idea.

Hope that helps.