Wired Keyboard:
+ numeric keypad
+ no batteries to replace/recharge
+ right-side Control key
+ larger Control, Option, Command, Space, and Arrow keys
+ seven more Function keys (F13-F19)
+ Page Up, Page Down, Home and End keys
+ adds one USB port (uses one in back, but gives you two up front)
- wired
- it's wider
Wireless Keyboard:
+ wireless
+ it's narrower
= frees a USB port on the back of your iMac but gives you none up front
- no numeric keypad
- missing and smaller size keys (see above)
- needs batteries every so often
For me, the wired keyboard is the winner. I never feel the need to move my keyboard more than an inch or two, so I don't really notice the wire.
Not so true of the mouse, however. I've used the Apple wired Mighty Mouse and a Logitech MX620 wireless mouse each for a year and half, and vastly prefer the latter. The mouse wire was a minor annoyance and I actually liked the Mighty Mouse's tiny scroll ball, even though it required a quick and easy cleaning procedure fairly often. No, for me the big problem was getting reliable right-clicks out of it. Also, I now realize what I was missing as it doesn't have any extra programmable buttons like my Logitech. The MX620 and its configuration software has worked flawlessly for me under OS X Leopard and Snow Leopard and I love it. I chose the Logitech after trying the Magic Mouse in my local Apple store, and glad I did.
Good luck!