I have owned the 11" DD coil for the F2 and the NEL tornado, which is a 12x13 coil. They both were my favorite coils for the F2. I've never had the 9x5 NEL, but since that is comparable to the 11'DD, I would stick with the stock.
I did have to ship my Tornado back for repair and they sent me a brand new one back before even receiving mine. I did have to pay shipping to the Ukraine though.
If you were like me though, and get hooked on Metal Detecting, you will never be fully happy until you upgrade your machine. I upgraded all the coils and it still let me wanting more.