I absolutely love both of them!
Overwatch is much more fun if you have friends you can roll matches with. It's got more of a fun, cheeky feel, and sort of feels like Street Fighter mixed with Team Fortress 2.
Titanfall 2 is tremendous, and an icredible experience as a whole. The multiplayer is fast paced, and the dynamic of the pilots movement, as well as that of your Titans is completely unique. It is really really unfortunate this game didn't come out earlier in the fall, instead of sandwiched in between Battlefield and COD. It's seriously a tough choice for me between Titanfall 2 and Doom for shooter of the year.