It depends on the content type you have.
If you focus primarily on video and pictures, I would say Instagram.
But if you are promoting a blog or a website that has different types of media on it, I would suggest Twitter if you are clear about the community you're trying to reach.
Twitter is really a tool not for direct traffic but for niche exposure.
In other words, people who are obsessed or specialized in certain niches use Twitter to find the latest and greatest updates in their field.
And if you get the eyeballs of these influential people, not only can you get more traffic, but they can also expose your brand to their existing followers.
Think of Twitter as an influencer marketing tool instead of a direct source of traffic.