I can't tell about Dota2 as I personally hate moba games.

but I ll compare it with tf2 as an fps shooter.
tf2 is more rewarding. its way harder to master and get good at it.
its funnier playing it. and it just got a competitive update.

playing team fortress games since 2000's I think overwatch is a nice casual alternative to tf2 but in no means a better game.