Don't really see this at all, and I am a big fan of Miller.
Kreider gets a lot of **** on here for being inconsistent, which he is, but Miller hasn't been the model of consistency himself. He had a very good year, but nearly half his goals came in an eight game span.
I think both Miller and Kreider are a big part of our future, and will end up being fairly similar in terms of their offensive output, but Miller will never have the game changing ability that Kreider possesses. Unfortunately, it doesn't come out often, but when it does, Kreider is a force to be reckoned with. I was watching some 2014 playoff highlights recently, and Kreider just had some absolutely insane moments. I haven't seen that from Miller yet.
That said, Miller is great, and I'm fairly confident he can push 50 points this year. I think Miller is smarter offensively and has better hands, but he needs to shoot the puck more and I'd like to see him be a bit more physical. Of course, he'll never match Kreider's speed and size, but he's a lot more slick with the puck.
Overall, I think it's too close to call on most fronts. If I was forced to trade one, though, it'd be Miller. I just think that what Kreider brings is nearly impossible to replace, even if he "only" puts up 20 goals and 25 assists a season.