I wanted a left eject upper cause even with an ambi safety and charging handle, when shooting suppressed, the gas cutting across your face and getting inhaled sucks. Most of it's mitigated by an H2 or H3 buffer but still, having brass eject away from your face on the same side as your cheek to stock is the way to go. If you're not shooting suppressed, really don't need a left eject upper - just make sure to fine tune your weights until brass spits where you want it to go and not on your nose.
Aero did me dirty when I ordered an upper from them, zero communication, took my credit card payment and sat on it for about a month and I had to do a chargeback. I found some small shop boys in Montana to build me a custom one to spec. Very well assembled with a lefty toolcraft nitrided bolt. It says everything's out of stock but get in touch and tell em what you want and they'll make one for you.