It depends on niche and quality of content also the time when you are sharing. So provide useful relevant content on StumbleUpon and Tumblr. Take a look at the article mentioned below. It will help you to understand more. http://www.quicksprout.com/2012/04/24/the-secret-to-getting-highly-targeted-traffic-from-stumbleupon/
I use both of these, but they are different from each other. Both are the same by how you can gather followers and every time you post something they will receive it, which is good and can generator quite a bit of traffic. With Tumblr you will only get traffic from others on Tumblr or from search engines once they index it, but remember that Tumblr is a blogging platform and Stubleupon is basically a social media platform where users share websites and pages. I hope this has helped.
For me traffic from StumbleUpon is a junk... I've never got a single sale from SU.. Well, for Tumblr, I often use it for Tier1 backlinks for my main websites, but it's pretty good for traffic too.. But you should already have a popular blog with 1k+ followers (which is not so easy to achieve in a short period of time!)