Kucherov spends each off season training and evolving his game - he's never the same player he was the year prior (very similar to Crosby). Hell, Kucherov changed how Tampa's practices go and clearly, you can see the results of it.
McDavid hasn't evolved whatsoever and it shows. I do believe that while not being the best in the world, he will almost certainly have better individual career compared to Kucherov. I think he'll be rather consistant / stagnant (90+ points each year) for the majority of his career whereas Kucherov will have a much higher peak but lower valleys when its all said and done.
If McDavid wanted better line mates, he shouldn't have signed for 12.5mil and left some room. Not Kucherov's fault that he took a discount to better his team.