Kane is currently 32 and has 403 goals and 1,074 points. Presumably he'll push those totals a bit higher by the end of this season. My career forecast model suggests that by the time he's 32, Matthews is on track to have more than 600 goals and close to 1,150 points. So, strictly in terms of regular season production, there's a credible argument that Matthews will be ahead of Kane - especially if you give goal-scoring a premium.
However, I'm voting for Kane for two reasons:
1. There's inherent uncertainty in any long-term forecast. When two players are relatively close, I'll take the one who's actually put up excellent numbers through age 32, rather than the one who's merely on track to. A single bad injury can seriously alter someone's career trajectory (Stamkos is a good example).
2. Kane has an extraordinary playoff resume - roughly on par with Sergei Fedorov, Joe Sakic, and Sidney Crosby. Not to say that Matthews can't or won't become a great playoff performer, but I'll go with the player who has already established their reputation, rather than someone who merely has the potential to.
All that being said - this is a closer comparison than I initially thought. By the way - I don't think Kane is the greatest American born player. He's hasn't yet surpassed Chris Chelios or Frank Brimsek.