I'd been using an unbranded (only inscription on it is Pro Terra) with an ergonomic parted in the middle design for the last 10 years. Broke down 4 times, could repair it 3... Shame, it had great size (rather large), keys had good movement, and overall felt comfortable once you got used to it. My advise, look for good ol' keyboards (like the amazing IBM Model M), known for better construction, classic layout (specially if you move arround several computers with high frequency).
What I'm trying to say, older models work better for me, because of their overall "feel". But, like most thing are, it's more a matter of personal taste, than anything else. You'll have to find out by yourself wich one fits better with your hands.
Edit
Ok, so I've bought a MS Natural Keyboard 4000. I'm in love. It works great and shares the same layout that my old unbranded keyboard did. It is surprisingly light, and has a lot of function keys that I rarely use (The exception being the sound volume keys).
Definetly, this is a good and comfortable keyboard, so I have to get on the bandwagon of people recomending it.