Aubergine 

A couple questions. First, have you looked at any other bikes than the two Specialized models to see if they would be suitable? And second, what sort of riding do you hope to do after your trip with your son is finished? And third, depending on what you think you want to do after the tour, why not think about two bikes? One for the tour and then one for riding when you are at your planned weight?

Now, my perspective is as a rider of classic bikes. All of my bikes (even the new ones) are steel framed. Some of them are racing bikes but a few are set up for touring or carrying loads. If I were to take a long trip even a credit card tour, I would head straight for a bike designed for comfort after hours in the saddle. For me, comfort comes from a good saddle, wide tires (not *that* wide; 32 or 35 mm is fine) and a bit of spring in the frame. There is no way I would choose my Masi or my Stevenson criterium bike. But after the tour? I am sure glad that I have the Masi, the Stevenson, and my Rodriguez for fast riding. So, perhaps your best choice is to find one bike for the tour and a second for simple enjoyment.