On wheel size, AFAIK the primary difference between models of dynamo hubs are their various spinning resistances, not their durability. Someone should chime in if this not the case because, as Yan said, your front hub is not something you generally have to or want to have to worry about. Spinning resistance on most dynamo hubs is almost unnoticeable. I am guessing you won't be able to tell the difference between the two hubs during a loaded tour. I can't really tell the difference between off and on with my Shimano hub when riding loaded or unloaded, nor do I feel it slows me down at all, and it is 71 model, I think. So, assuming both hubs are equally durable (and I think that should be your first concern when making touring-focused choices), you should pair the cheaper hub with the better rim rather than the reverse.