Hi Rusty, I've never used a Nugget Finder coil I didn't like, solid, well built and a separate shaft mount not a moulded part of the top plate so no flex or cracking there.
Firstly though, what model NF, eg: advantage? evolution? or early series white coil and what if you don't mind asking is a good price?
14x9 is a very sensitive coil and great for learning to pin point with, usually a bit easier to learn than a round coil.
14"+9" = 23". So 23 divided 2 =11 1/2" so not much more surface area than your 11" mono which is a disgustingly good coil to start with and would still debatably just ping it in depth ability, though ground coverage on each sweep the 14 x 9 might hold a bit of an advantage.
Me? I'd forgo the 14 x 9 and find a 15" round NF evo.
Cheers and welcome.