Instagram or Twitter?

I'd say use both too. Although in my opinion twitter is a lot better to connect with people. And also I'm not sure if this is true or if I'm confusing instagram with snapchat right now but I think I heard your posts will go away at some point, I dont really like that
I absolutely use both. It really depends on who your audience is and where they are. I also use Google Plus and Facebook, Pinterest. You could even start a blog and promote your videos through posts.
I use Twitter more for making connections with people as it is much easier to reach people on that than Instagram... at least that's been my luck. You can promote your stuff on Instagram but it's probably a bit annoying. I do make a post on Instagram with either a thumbnail photo or a short clip from the video. I think the most important thing about these platforms however is hashtags. You never know when people will be searching through hashtags. I know I'm not one who really searches through them much but there are people out there that just follow certain tags. I did some E3 videos where I tagged a lot of the games announced during this year. I got a lot more random likes on the photos but I don't know if that translated to views. But hey, at least I know people were seeing the posts!

Don't be too spammy with promoting though, that can really make people want to unfollow you quickly. I know. I follow some accounts that just do it too much and have been very tempted to unfollow them because of it.
We have both, but we find Instagram is easier, and we personally like it better because we have more creativity to post cool pictures.

Julia and Claire
I personally like to use both platforms. This depends on who and where your audience is. Also, use the platform that you are strongest with and it's also good to practice with the one you are not so good with. I am seeing growth with both accounts. They are free promotional tools, so why not use them both?
I work in social media marketing and I can tell you that as a creator, Instagram would be your best bet. Instagram is a wonderful platform for creators because it allows both photo and video along with no restrictions on character count.

However, Twitter is very useful as well when it comes to building your brand and spreading awareness of who you are/what your channel is about. Essentially, you should be using both. If you really want to brand your channel separately from yourself I would suggest separate social accounts on each platform that match your channel branding.

Short answer: both are very helpful in growing your brand/channel.

Hope this helps! :)