For us, it was simple. We got the lightest one available which is the r-pod152. We figured that the lighter it was, the more frequently we would use it both from a pain-in-the-bum-to-tow standpoint and from a cost standpoint as well.
We used it every other weekend from June through mid-October last year not including two-week-long trips to the Fingerlakes region, Bar Harbour ME and VT. Some of the areas we went to were very hilly and we were glad that we didn't get anything larger/heavier.
We did meet a number of people who did longer trips and were mostly full-timers and we thought that we may go up to a 171 with bunk mod or a 173 at that point. However, we do a lot of things outside the pod during the summer months so the interior space isn't that important right now.
If we traveled during colder months and were not as active, we would go a bit bigger, but not that much bigger, again due to cost to tow and effort to tow factors. We just would rather get out more often.
From what I remember from researching before we bought in 2010, the MPGs were quite a bit heavier than the r-pods for comparable models.
Good luck with your choice. I'm sure whichever one you choose, it'll be perfect for you :)
-T