I think Dier is significantly better. With and without the ball. In his in game intelligence, and execution of passes etc.
Henderson gets a rough ride relative to his station. He is, IMO, pretty well average. "decent" is about as best you can describe him. He is really quite limited IMHO and really only good for certain roles.
At Sunderland he was their best player, by virtue of seeming actually competent/proficient. Relative to the club people were like "hey, hes ok".
He is not, however, anything like good enough for a club of Liverpools aspiration. It baffles me they were interested, let alone spent (for the time) good money on him, and since he has somehow become their captain. Its mental.
And, of course, its because of that he is in the England squad now.