Hmmm..., what are you looking for in a car? The 2021 K5 will have all the latest technology when compared to the Stinger, it will have a larger interior, be more fuel efficient (1.6T vs 2.0T or 3.3T if you go that route) and if the rumor is true, could come with an AWD system. In my household, we have a 2017 Optima Hybrid EX, which I have lowered and put some wider tires on. It's spacious and comfortable, drives and handles well for a mid-sized sedan and the trunk space is quite nice. The 2019 Stinger GT2 AWD is a whole other animal, fun to drive, can carve corners way better than the Optima, is quite comfortable in it's own right but with less interior space than the Optima and space beneath the hatch cover isn't as large. However, for normal daily cargo carrying, it is fine. I am not carting around folks most of the time and the space is fine for me.
It will really depend on your use case but the Stinger is a great car with the 3.3T and if economy and space isn't a big thing in your book and the deal is really good. I would be taking the Stinger.