I'm also working on a similar issue. One of the main difference that may interest you is the performance. I just found a blog post from the developer of "perfect-api" comparing his framework to the 2 others. http://blog.perfectapi.com/2012/benchmarking-apis-using-perfectapi-vs-express.js-vs-restify.js/
A comment states that Restify is doing much better now.
From my (limited) experience, I would say that express.js is easier to use than Restify and provides some nice tools as connect.js which is exposed. For example, this will avoid you the pain of writing a static file server even thought Connect can still be integrated with Restify.
On the other hand, Restify will make some common API tasks much easier. For example, it provides a throttle engine along with an API conversioning system which is really necessary in an environment where you deal with clients you do not control.