The G37 is one of the best deals on the used market in my opinion. The negative (deserved) publicity that Nissan and Infiniti get now drives down the prices on these I think. The G37 is a solidly built car and has good reliability scores on CR and TrueDelta. They also were pretty quick and fun to drive.
At one point when the Genesis Coupe came out, I remember reliability scores were bad. I don't know why because the Genesis sedan was reliable. TrueDelta shows reliability for the Coupe as very good, while CR doesn't have enough responses to make a conclusion.
The Mazda6 is a great and reliable car, but you will give up power. If you get the 2016 and newer with a touchscreen, those are the ones that are upgradable to CarPlay and Android Auto as they use the Mazda CONNECT head unit. The 2014-2015 has a radio you would need to have replaced with an aftermarket unit to get these two features.
I would knock the Genesis off the list, but if you find a good deal on one I wouldn't hesitate to test drive it.