There's one thing going for Marner over Rantanen this summer, and it's that I think his game might end up ageing better. Rantanen is super talented, but I can see the physical drop off affecting him more.
One could argue that Marner plays PK, while Rantanen doesn't. Fair, but I'm of the mind that you can find plenty of good middle/bottom-6 players to do that for you. I'd rather have my superstars more fresh for 5v5 and PP, and not have them do PK. But, this is a very divisive topic in the league. Teams have different philosophies. And if Marner is that much better than all of your other forwards on the PK, you do what you gotta do.
Outside of that, it's not really a close debate. Rantanen scores more goals, he scores more points overall, he has a more physical game with a bit of a mean streak when he happens to be on that mood, and his playoff pedigree speaks for itself.
All of that said, if both players are going to stay with their respective teams, Marner will get paid more once again.
Avs "no injuries" just missing their captain for the third season.
After like the first week of the season, the Avs were missing 6 of their top-9 forwards. Yes, the most important players MacKinnon and Rantanen were healthy, but they basically had to play with 4th liners while the bottom-6 was filled with AHL players. The injury situation with forwards was actually more severe with Colorado than it was with Toronto.