I’ve already seen a friend of mine who has built a great audience on Vine switch to Instagram for Video. As big as he’s grown his Vine following, his Instagram following is even bigger. It’s a no brainer for him to switch over.
Playing with video on Instagram, I really miss Vine’s loop feature. I’m surprised how accustomed I’d become to expecting that and how much I enjoy it. I also find Vine loads faster, but I’m sure Instagram is getting hammered today.
Ultimately, I think it depends on which social network a person shares on primarily. If they post on facebook, they’ll use Instagram. If they use twitter primarily, they’ll use Vine. The market is probably big enough to support both.