Just my experience, but I recently switched from the City Contacts that were stock on my LHT to the Schwalbe Marathons and it feels like I'm riding through cement. Their reputation indicates that they are in fact good tires, but the difference in rolling resistance was noticeable for me.

As far as just replacing one tire, I've always adhered to the rule of putting the new tire on front and move the front to the back. The reasoning being that it is more dangerous to have a tire blow out (loss of control) in the front than in the rear. There is some assumption of bombing down hills at Mach three roadie style in that line of reasoning, but it seems to make sense regardless.