Not really what you want to hear, but all of them are. It really boils down to how much space you need/want. Each Airstream is fully self contained, has connections for external supply for water, power, sewage, phone,TV/Sat and are prone to follow to same laws of physics as anything else on this rock. We are talking an RV here, so although some features may be better not to have in terms of insulation from the outside (skylights, vista views, wrap windows, etc), the reality is that it's a RV that is not airtight by any means and has significantly less "R" value than that of a house. In winter with any unit, expect your LP costs to go up....larger units have larger heaters and more heating space. Summer, if you are paying a flat rate, it's not as much of a concern, but again, larger units typically have larger A/C units, some even have two and once again, larger units can take more to cool on very hot days. So to me the better question is how much space do you need and from there how can you squeeze the most efficiency out of the unit that meets your needs.
