In terms of personal preference, Mantine. In terms of who is better in battle, I dunno, possibly Skarmory slightly more?
I probably have used Skarmory more often, although both of them were in recent times. I did an AlphaSaph nuzrun two years ago and had a Skarmory there and he was pretty useful until a Medicham destroyed him with Reversal during a trainer rematch shortly after we beat Primal Kyogre. Then my next Skarmory was in Complex Crystal, a ROM hack and he survived until the end even if added only shortly before the last gym.
Why I like Mantine better, I really don't know. Possibly because I just like it's design or something. I only recall using it like only once ever, possibly because WHY CAN'T IT LEARN FLY!?