Balsamiq is simple and easy to use. Once you figure out how to install it, there are many ready-to-use UI elements available as a standard part of the software. There is also an Atlatissan Wiki plug-in available for everyone (even managers) to use.
Axure is more flexible and powerful. Advanced UI elements are also available but as separate downloads. What is great and pretty unique about Axure is that you can make interactive prototypes to demonstrate all kinds of functionality. You can them send these to your clients – all they need is Internet Explorer to simply click through your (multi-page) demo. I also like its ability to produce Word docs and detailed functional specifications – very useful!