They're vastly different genres and it's all up to personal preference.
I've played through Dark Souls 3 and even though the first few days were exciting and fun, it soon got stale for me. But you may have a different experience than me because I didn't play any previous Dark Souls titles.
I've also played Overwatch and have thoroughly enjoyed myself so far (have around 84 hours clocked in so far) and keep continuing to play the game. Then again, the entire reason I purchased Overwatch was because I used to play TF2 a few years back and this looked like a much improved and refined version released by Blizzard.
Both games are great and you'll have fun either way.
If I were you though, I'd buy Overwatch.