Havent played either but from what ive seen:
tf2 has a balanced roster and a mix & match cosmetic system, but it has whiny preteen brats
Overwatch has a diverse roster and amazing skin cosmetics, but has whiny manchild brats
Blizzard just wanted to give valve competition on the mmofps market

Every playable character.

Yup.
Though I only play with my sister~ =w=
And I don't like to bring my gender out there in games, it never ends well xD
It's always 50/50 with people trying to go full "M'lady" and people who start harassing and trying to make me as uncomfortable as I can. o_o

I have a few in-real-life buddies and some really nice peeps who trade with me, but other than that, people seem like a girl playing TF2 is the end of humanity.

As a TF2 player, I can confirm that most players are whiny preteen brats, young children, annoying tryhards that love killing in Casual. There are still a few good people though however. Overwatch seems decent, it's certainly gotten more attention than TF2 (before Jungle Inferno)

Tf2 all the way… overwatch definitely couldn’t compete with tf2

I can't believe people are still having this conversation, lmao
And the thing is, it always seems to come from the TF2 side of things. It's like an insecurity of TF2 players that whenever they spot an even remotely comparable game, they try to prop up TF2 as some unkillable titan of gaming that obviously nobody can destroy, ever.
"Y-yeah, TF2 is awesome, guys! Wh-wha? The bots? N-no! They've been mostly dealt with, dude! Up? Dates? No, no, we don't need those, haha, it's fine, trust me bro! Everything is awesome! The fanbase definitely isn't going insane! We're FINE! FINE!"

Overwatch players don't give a fuck. Neither should you.