I'm building a -7, on the wings right now. So please pardon a -7 guy barging in on y'alls -14 forum
I tossed this same question around a few times myself. Van's shipped the warner kit by default, so there was no "monetary" influencing factors. I ended up deciding I would go ahead and install the stall warner, in addition to doing AOA eventually in my panel.
My reason:
Redundancy. The stall warner is a very simple, but very effective little system. It's pretty robust.
Second, think of it like this. AOA is like the gas gauge in your car. It tells you how much "lift" is remaining on your wings (sort-of). Unless you EFIS supports it, it doesn't explicitly WARN you on an impending stall. Its sole purpose is to indicate how much AOA you have.
Now, the stall warner is just like the "low fuel" warning light in your car. Its SOLE purpose is to EXPLICITLY warn you that you are running out of fuel. Same for the stall warner, its sole purpose is to EXPLICITLY indicate to you that you are very very near the point of a stall by a few knots.
It's only gonna cost me a couple hours of labor to install it, so I figured I might as well install it.