This is a perennial question amoungst UX/IxD/UI folks. I haven't used Axure and confess to be suspicious of its hype. My secret weapon is still: paper. I know you said digital, but in my experience all digital prototypes have the inherent weakness of needing a nontrivial investment of time and brainpower JUST TO MANIFEST the thing, to say nothing of making it well designed, or even just coming-out-how-you pictured-it.

I never have this cost/risk with paper. And ALL development participants (users, devs, designers, businessy types) can use pencil and paper expressively.

Bottom line: keeps the focus on the design and the user task and not on the clumsy medium. Helpful?

If you twisted my arm: I'd either say vanilla drupal instance with commmon modules (views, etc.), rails stack with widget libraries or ...Powerpoint.