Both have their merits and uses. I use Instavideo when I want to capture something on the fly but don't necessarily want to /loop/ it, since Instavideo on its native app doesn't loop the resulting video. I also use it for when I want something that lasts more than 6 seconds (up to 10sec. if I do it in one take).
Vine is great for stuff that's looping, especially things that derive its point of entertainment through sheer repetition. I used to use Vine for the same reasons why I used Instavideo (and I still do sometimes), but I've recently learned that something work watching on Vine is mostly stuff that would be entertaining when it's looped, which is a downer for info videos or little oneshots (though that's not to say that it's not feasible to do as such; depending on the material shown like 13 Sonic and Knuckles cartridges in a 32-part series, it could possible work to one's favor)
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