Mac Without a Doubt....
I don't know how many presentations that I've made over the last couple of years but I can't live without my Mac.
Keynote comes free with the new versions of OSX..AND....If you have an iPhone, you can install Keynote on it as well. The beauty is the fact that the iPhone will pair to the Mac and you can use your iPhone as a presentation 'tool'.
When you are running your presentation, you are able to see any notes and preview the next slide, right on your iOS device. You can also use the iOS as a 'whiteboard' application and draw or markup the slide as you are talking through the presentation.
Its a great pairing and I couldn't live without it.