I used to fly KC-135s and I have some friends who've flown C-5s. I'll caveat that this is all AD gouge and may be stale. The cargo mission involves a lot of flying from point A to point B, with a wide selection of As and Bs to pick from. The tanker mission involves a lot of taking off at point A then refueling someone and then landing back at point A with a less wide selection of As to pick from.