The huge disparity in their contracts is the only reason to even consider this from the Leafs POV, even as a purely hypothetical. For the Sens, the opposite reason. Plus, if Ottawa ever considered moving Brady I'd imagine the priority would be a center coming back.
But sticking to these two, Tkachuk is not anywhere in the ballpark of the player Marner is. Brady brings a different skillset, sure, and the Leafs could use some more toughness / physicality in their forward group but over the past 4 seasons go ahead and count the number of players that have a higher PPG than Mitch. It won't take long. Kills penalties too. He's really good.
Mitch is an exceptional talent that unfortunately got paid too much and is part of an under-achieving (playoff) group. But last time I checked, he's helped his team get into the playoffs the past 5 seasons and Toronto has a legit chance at winning the toughest division in hockey this year. Brady doesn't have anywhere near the level of teammates that Mitch does obviously, but he sure hasn't elevated them either in any sense before and since getting the "C".