URL alias: Which canonical URL should I use?

After having read this post, I want to add a rel="canonical" header line in my webpage. This webpage is stored at http://ift.tt/1xZDJRI , but there's an alias or redirect (I don't know which term is ppropriate) from http://ift.tt/1DS49fv , and I've been told to use the latter as it is the "permanent" URL for reaching my pages.

Which URL should I put in the "canonical" header line?