I wouldn't say "easily", but I'd rather have Marner.
Seems more of a complete player to me. Better two way player and better 5 on 5 player.
Rantanen does have the size advantage, but in today's game, Marner's intelligence on the ice more than compensates for it....and Rantanen isn't exactly a bulldozer on the ice, despite his size. Sure he will use it, but a physical specimen on the ice (something along the lines of a Lindros) he is not, so I think that particular advantage over Marner isn't as grand as some may think.
Marner also has a propensity to relieve the opposition of the puck quite often and again, he does it with smarts and precision skill...and you can bet when he does, he already is thinking ahead to where he is going to dish that off as he looks for a streaking linemate.
Either player is fantastic to have and build a team around, but IMO, Mitch Marner is the complete package and just seems to have more tools in the box.
I think when both their careers are all said n done, Rantanen will be known as the great sniper who was always a big handful for the defense to contain, but Marner will be known as the guy who could do it all at a high level and the guy who truly elevates those around him.
And this is coming from a Lightning fan who doesn't particularly like the Leafs.