As a fan of both teams I would say that:
Nylander and Mittelstadt are about even. Both have a lot of potential, and some flaws. Yes, Nylander has proven himself in the NHL, while I am not sold on his compete level, while CM has only played 6 games, but a pretty good first 6 games (although there were some holes, and he got a little puck luck). Both had really strong WJCs during their D+1. If you gave me a choice between the adding one of the two to "my team" it would take me a bit to decide. If I was playing it safe I would go with Nylander because he has proven himself in the NHL. If I was taking a risk I would go with CM because I think that he has a higher ceiling.
I rate Dahlin higher than Marner. Again Dahlin hasn't played yet, and D are harder to judge how they will turn out, and I think that he has become a little over-rated by some on these boards. Still Dahlin is the real deal. Best D prospect I have watched in a long, long time. Marner is great but he is not Dahlin. If you gave me a choice between the adding one of the two to "my team" it would 100% be Dahlin.
Matthews I place significantly above Eichel. I know that many will say they are close, but I have watched Eichel from his first NHL game, and Matthews starting about a week into his NHL career and I have not missed many of their games since. I also expected Eichel to be the better of the two and am from Buffalo. Eichel is great, amazing at transitions and extremely exciting to watch. But Matthews is way better. If you go by stat watching, Matthews leads comfortably. If you go by pretty much anything else Matthews leads by even more.
First two years:
Eichel: 142 GP 48G 65A 113Pts
Matthews: 144GP 74G 58A 132 Pts
So overall I would rank the two threesomes pretty even. Tie goes to the team with the best player among the 6 and that is Matthews.