This thread is based on an excessively dichotomous view.
There is more to diplomacy than "diplomacy and tactics"
The best categorisation I can come up with is:
Diplomacy - building trust
Diplomacy - convincing
Anticipation
Tactics
Strategy
Though I'm sure I'm missing something there. The usefulness of the skills depends on who you are playing. Against weaker players in tactics and strategy, being convincing is more important, but against strong players, who tend to be immune to persuasion, it borders on irrelevant, for instance.
I used to be heavily dependent on my tactical ability, and then use that to be more convincing arguing for a particular course of action, but over the past couple of years I've evened out a lot, and am now oftentimes sloppy about tactics, having done all the groundwork.
I can remember playing against MarekP when he was at his prime in 2008. His ability in the anticipation, strategy and building trust was legendary. He didn't really try to convince you of anything, and his tactical skills were just ordinary-good, but being brilliant in those 3 areas, he managed to win games without stabbing anyone. It was freaky.