The Q50 has better tuning support, and you can get an LSD (aftermarket) for RWD and AWD, which you can't for the Stinger (in AWD), at least not yet. The transmission isn't great, but stout, the AWD is proven, the brakes are generally good. And, you can EASILY get $8-10k off MSRP. Plus, they have almost ZERO MF for leases, so you can often get a fully loaded lease for $500-600 - less than I'm paying for 24-months of the GT1 (but I'm planning on doing buyout after a month or two, once I'm "sure").
I was all set to buy a low miles 18 Q50 sport AWD for $27K, but then I found they have very narrow / uncomfortable sport seats (they made my left leg/hip shoot with pain for several days after sitting in it for 10 minutes), which was a deal breaker for me, obviously. Stinger so far has been great. YMMV.
Q50s are a similar "bang-for-buck" as a Stinger, IMHO, and the dealers are generally considered MUCH better than Kia - Infiniti is Nissan's "Lexus" line, so they are (near) luxury dealers, not mostly-sub-prime like Kia. The infotainment is very outdated, but according to most people does work ok, and you CAN get Tesla-like replacement units with varying levels of performance / user satisfaction. If GO instead of SHOW is your thing, the Q50's not a bad bet - search youtube for a non-sport version that dropped $4-5k in fuel pump + injector upgrades and with E85 embarrassed Lambos, Porsches, etc in the 1/2 mile. If you like the fancy infotainment,
android auto /
carplay, coupe look, newer chassis, etc, the Stinger probably wins out. For example, you can't get cooled seats on the Q50, which seem really nice so far on the Stinger. However, I *WISH* we had Ecutek or similar available for tuning.