DDG is on massive wages and his value is only going to start dropping. The guy's 31 and his command of the box has barely improved in recent years. Whilst his reactions are still good, they'll be the first thing that goes. When they do, it'll leave him with nothing to lean back on.
From a purely business point of view, this is the moment we'll look back on and say 'we should have sold him then'.
Henderson has a lot of qualities that seem to suit Rangnick's style. Even if he doesn't turn out to be world class, he's at least young enough to retain his market value for a few more years.
I think the sensible thing to do is to sell DDG for a decent fee, and give Henderson a chance to prove himself as a sweeper keeper. Then if he can't make the grade, sell him as well and buy someone else.
This is the Rooney moment in Moyes' first season. By failing to sell when we could, we ended up keeping him beyond the point he was productive and his huge salary meant the manager was forced to play him every week to justify it to the board.