Miller is the player that seems to visibly improve season by season. First year, he was pretty bad all over the place. Constantly out of position, pinching at the wrong times, forcing really bad passes and plays, not backchecking hard enough. The following season he ramped up his physical game and his offense came along too. Although his PIM last season was double that of the previous season, it felt like he took a lot less unforced/stupid penalties, granted they still occurred. Like silverfish is saying, his production will most likely take a slight hit next season, but I suspect we'll see better defensive play from him.
Kreider looks more and more lost every season he's spent in the league. It's great he can still put up points in the mid-40s while looking like he has no clue why he's there, but I still have a lot of trouble figuring out what Kreider has gotten better at. He's not a particularly better passer or pass receiver, he's not a particularly better shooter, uses his speed less, uses his body less. I really don't recall much of anything Kreider did until the last ~10 games of the regular season last year. I remember my jaw dropping around game 75 seeing Kreider blow past someone because it was something he hadn't done all season and it was crazy how fast he was.
I'd take Miller over Kreider if there was a choice to be made, but I'm just happy we have both.