I am another lefty who PREFERS a standard upper! By intent, or accident, Mr. Stoner designed an excellent lefty rifle! (Well, actually, only after the deflector was added, behind the ejection port.) Righties have been inventing such gadgets as the B.A.D. Lever to convert the original AR15 to be more rightie-friendly.
I know this puts me in the minority among lefties, but I actually dislike ambidextrous selectors; they get in my way. Obviously, this is going to be a very individual thing. I prefer the non-ambi Rock River selector with a star-shaped nub on it, which is perfect for the base knuckle of my left index finger to engage, Kyle Lamb-style.
Having said all of that, well, I prefer a Mini-14, as it is an even better lefty rifle.