TF2 is 10 years old now and F2P since 6 years.
It got massive updates and overhauls throughout and has a unique artstyle (Overwatch also, no debate).
But this particular FPS game genre was born with Team Fortress, TF2 being it's high successor and current representative of the "series" (until Valve decides to release TF3 lol
I won't say Overwatch is a copy of TF2 but it sure is the most similar among all the popular titles of this genre.
Gameplay and graphics in Overwatch may be more comfortable and prettier than in TF2 but both have a strong focus on having a own unique style, both in gameplay and visual appearance.
When it comes to funds and experience both studios are probably the biggest. But Blizzard? C'mon, it's huge!
Valve has the main focus on their Steam Client. Blizzard makes and updates only games.
Overwatch is a great game, but I wouldn't describe it to be X times better than TF2.
lol I really wrote all that now
Have a good day guys.