It depends, who is your actual target audience? If it's a general anyone and everyone, gif is the way to go. If it's a more technical audience, who probably know what webm already is, then webm may make more sense

And from a more marketing side of things

If you went gif first and five months down the line announcing support for webm makes you look innovative and will probably get you posted to a handful of subreddits and the front page of hn.

If you went webm first and five months down the line announce gif support, you probably won't be able to achieve the same results.