OEM means Original Equipment Manufacturer. Its typically cheaper because of the lack of documentation (Treeware) and CD. Its usually purchased by companies that dont need the "extras" (i.e. large corporations to upgrade end end user employes, or companies that build computers for clients. Its chaper and they offer tech support so the end user doesnt need it.)
I usually go with the OEM, it IS cheaper and I can get drivers online..
My optical drives and hard drives were OEM.
I hate "natural keyboards"
but MS is coming out wiht a Vista keyboard that looks nice for a natural.
On the topic itself, I would say go with the first one. However, if you are up for suggestions, mine would be the Saitek keyboards or the G15 from Logitech. I like the Saitek over the logitech, especially the nice backlit keys effect.