If it were up to me I'd give it to Tavares, he's been a captain before and he's well established in this league. I fully believe Matthews will be the captain of the Toronto Maple Leafs, its not really a question of if but when. Give it to JT, he can wear it for 5-6 years and once he is 32-33 ish Matthews will still be just 25 and in perfect shape to take over the captaincy, more mature, seasoned, dominant etc.
Doing it this way would give Matthews a first hand opportunity to see what its like to be a captain of an NHL club. This is something he's never been exposed to and it lets both of these great players say they've been the captain of the Leafs.