I have owned both iPads and after hard day in and day out use the battery is still only needing charging every few days or so.
By the time your battery shows any signs of not being the same capacity, you will be getting the iPad 4. If you feel better getting the warranty, do so, but Apple is not going to fix this at your wish. It is only natural for there to be a slight decrease in performance but you would have to have a drastic decrease to have it covered under warranty.