Kind of depends on what you'll be doing with it. I've got the base 27" 2011 imac at the office and the 2.4 Macbook CE* at home. For ordinary consumer stuff (browsing, emails, word processing, windows in boot camp,etc) I can't tell an appreciable difference.
If you're using software that can more effectively use the multiple cores, better gpu or increased ram of the iMac (games, photoshop, or running windows in a virtualization layer) there will be a noticeable difference in performance. Oh, you'll definitely notice a difference in screen quality!
Considering the historic yearly refresh cycle of the iMac, it should be a safe guess to say the 2011's will be halfway through its lifecycle before a new model is introduced. Whether you should wait or buy at that point is almost a subjective choice.
*You know, the ONLY Macbook to ever be released with a backlit keyboard...😛