MX Duo, definitely.
My wife got the set for her computer at work ($60 after rebates and coupons), and she loves it. I've got the Elite Duo at home and I'm quite happy with it as well. The mouse on the MX Duo is about the best you can get, and the keyboard is pretty spiffy as well. There are lots of very useful contols on the Logitech keyboard that MS still hasn't perfected. The Logitech dial-style volume control is a lot more intutive than the button-style MS version. The scroll wheel on the left hand of the Logitech keyboard is also really nice for serious two-fisted web browsing. MS tried it a while back, but they made their scroll wheel bigger and fuglier, which really ruined the look of their Office keyboard.
The keyboard is also quite comfy for extended typing sessions, which is really key. Like I said, my wife loves it and she does a LOT of typing in her job. Their zero degree tilt thing really does seem to be a bit easier on the wrists. My wife used to get carpal tunnel pain really bad, but with the MX Duo she hasn't been feeling it as much. I've not felt any pain out of my Elite Duo either for that matter.