Gareth,
Are you talking dual-pass duplex scanning or single-pass duplex scanning? If I were in the market for an all-in-one, I'd spend the extra bucks for single-pass duplex scanning. It's a game changer.
I first had it on my office's HP Officejet Pro 9025e and I couldn't believe how much better it was than dual-pass duplex scanning. So good, I bought a Fujitsu desktop scanner with the feature for home. I ended up getting a newer version of that scanner for my office, too.
Once you've used single-pass duplex scanning, you'll not want to go back to multi-pass duplex scanning. It's that good.
Robert