I prefer Axure, because it is easy to learn and use. You can find various help documents and videos on their webpage.
I recently evaluated it for my company and we now purchased a license. We design a complete prototype within Axure of one of our new products (Web-Client) before coding. It is very cost efficient to produce these mockups and they can be connected with a bit business logic, which ist highly preferred by managers to "feel" and see how the user will use the software.
We managed to notice many problems and got several new ideas before we developed the real product. Now we have a prototype of what we want to build and the developers can transfer the images to code. They save immersive time and energy because they don't need to think over every design aspect.
It is also multi-OS (Mac and Win) with one license. Sadly the license fee is a bit high and the software lacks an autoupdate process. If you are a member of the UPA you get the license 89$ cheaper.
You can download an 30-day evaluation version directly from their page. If you have further question I will be happy to answer them.