Hi Ian, I drive both, I have a 2006 Mondeo 2.0L TDCi (115bhp) and the wife has a 2007 Focus (the 2008 is only a face lift not a new car). I drive both regularly and as I said the 1.8TDci is easily powerful enough for towing, I have always found the Mondeo to handle better, is (obviously) more roomy and far more comfortable for longer journeys. I always feel that I am sitting too high in the Focus and I always get back ache. The Fiesta is even worse, in a Fiesta I feel that the front seats are tipped forward slightly making it feel like you are about to slide forward out of the seat. Horrible.
Given that there is a new Mondeo now which is supposedly even better than my Mk3 I would stand by what I said, it would be better to but a Mondeo. There is surprisingly little difference in purchase price if you go for nearly new rather than brand new.
I am surprised you think the Focus gives excellent fuel economy. I have always been very disappointed in how fuel economic either Ford (Focus or Mondeo) is as both cars give around 42mpg mixed cycle in solo driving. My older (2002) Mondeo 2.0L TDDi used to average 48mpg and my old 1999 Skoda Octavia TDi averaged just over 50mpg doing exactly the same sort of driving.