Well, I've never actually thought about comparing the two. From a user's perspective, I'd go with Tumblr. It deals with quality versus quantity. You have more posts of value on Tumblr than tweets on Twitter.
From a bloggers perspective, I'd suggest Twitter. The thing about Twitter traffic is the purpose of what you're actually doing it for. If you're simply looking for traffic on your site, it's the right direction. But if you're looking for referrals or affiliate leads to convert into sales. It probably isn't the best place. I've gotten some conversions from Twitter. But not any real quality traffic.
The one thing I would say is that in the comparison between the two, why not go with both? In the hunt to get traffic to your niche, the idea is to pull for all viable sources of traffic. Rather than one specific quality source. So be open to different sources of traffic. Even sites like Google+ that are somewhat fizzling out, are worth promoting on here and there.