Everyone is different, so it's not the sort of question that has a "right" answer. I have problems if the keyboard has any sort of tilt to it at all, so the thinner and flatter the better. Some people like the split keyboards, but for others (including myself) they're worse...They usually angle a little to the outside, and that bothers me a lot...Can't use a trackball either, for the same reason.
I recommend buying some inexpensive versions of each type and working with each for a while, to see which one causes you the least difficulty. I also recommend making sure the rest of your work environment is set up to minimize stress on your wrists...I used to work with a lady who bought a new keyboard every month in an effort to "fix" her carpal, when even the most cursory look at her desk would tell anyone that the problem was her hellish keyboard tray (which was at a 45 degree angle, and has this metal lip that she rested her wrists on...Made my wrists hurt just looking at it. Make sure you have plenty of room on your desk to rest your arms, and that your chair is high enough, etc.
Finally, I recommend getting some kind of hand exercise ball...not necessarily a squeeze type ball (I dislocated my right wrist when I was a kid and to this day that sort of exercise makes my carpal flare up) but something that's going to make you use your fingers in a manner different from typing. I use meditation balls...They don't cost very much, and they last forever.
