Panasonic all-the-way. I've been spec'ing them for bathroom and other ventillation needs (kitchen, laundry, fresh air) for the past couple of years and they've not failed to impress. Nearly dead-silent, and they're rated for continuous use, which I've yet to see any Broan/Nutone claim. In the one location, we've been using the Panasonic fan to draw fresh air into an electrical room, which it's been doing silently 24/7 for the past 3 years without fail. They also use 1/10th the electricity of what the comparable Broan/Nutone fans use, which should give you an idea of their engineering.
The only people I see thinking Broan/Nutone fans are any good are electricians who've not used the Panasonic fans. Shop around online for Panasonic fans... the prices on them are actually right in-line with the so-called "quiet" Broan/Nutone products, but the engineering on the Pannys is years ahead of the others.