Two piece ones that have a bend in them have more adjustability than the one piece ones. They are usually more expensive but I try to get these kinds. This way you can set them at the most comfortable angle for your wrists. Also if you drop your bike you may only have to replace the bar part and not the clamp.
These are one example:
http://www.fastfromthepast.com/servlet/the-725/Clip-dsh-on-Low-Rise-Handlebars/Detail
While I have no experience with these ones, I have bought stuff from these guys and been satisfied with the products and the quick delivery.
Dime City is great too supplier too, nice stuff and quick delivery and great rep and they give you a discount after you have made 100 posts:
http://www.dimecitycycles.com/catalogsearch/result/?q=Clip+ons