I can give you my opinion between the two.
I have put around 1k miles on the stinger with 500 of those being a trip to Indiana. Compared to 30k miles on the GSF over 4 years. I have a lot more seat time on the GSF. Also the GSF had 117k miles on it when I purchased it and 145k when i sold it. Like I said above I haven't really done too much spirited driving yet in the Stinger.
But here goes: 26~27k mile 2023 Stinger GT2 vs 117~145k 2016 GSF
Ride Quality--Stinger. The Stinger in either Comfort or Sport has a better ride. No question. My GSF was a 2016 and did not have adaptive dampers (available in 2017+). Stiffest riding Lexus I have owned (Prior SC400, GS350 F sport) The ride was good on smooth roads, but on bad roads not so much.
Handling--GSF. From the little bit of spirited driving I have done, the stinger does not feel as composed as the GSF. The stinger is not bad at all. But I had much more confidence pushing the GSF through the mountains I drive. I'll try again when the weather is more appropriate. Although I do prefer the steering feel of the Stinger.
Braking--GSF. Kia has good brakes, but Brembo 6 piston front and 4 piston rear with massive rotors, no contest.
Powertrain--TIE. This is Stock vs Stock. Both make similar Torque, but the GSF makes 100 more HP and has a wider powerband. I'm new to the TT V6, This thing is very punchy down low to 5500rpm, I love it! Off the line to 60 I'd say the Stinger wins 95% of the time. The GSF would come round the Stinger after that. The GSF has very long legs (7200 rpm) and that can a problem. To really ring it out you will be doing some serious speed. Around town the Stinger is more fun. (Fuel economy, if you care about that sort of thing, slightly better in the GSF)
Exterior Styling--TIE. This is subjective I love the looks of both.
Interior Styling -- TIE. Again, subjective both are great. However the GSF has better materials. nearly 10 years and 145k miles and the interior leather was pretty much mint. The Stinger is already showing wear on the leather at 27k miles. This could be from the previous owner not taking care of it or not caring about ingress and egress of the vehicle. Its not terrible but I notice it.
Technology-- Stinger. Lexus has always been late to the game with tech. All of the safety features (BSM, FCW, LCA, SA etc.) were there on the GSF but the Stinger does it better (7 years newer). And don't get me started on the mouse stick-- joypad thing to control infotainment in the GSF. The bright spot for the GSF here is the Mark Levinson sound system. To me it sounds better than the Harmon Kardon in the Stinger. Adding Poweramp EQ to AA in the Stinger helps, but the GSF sounds better.
Aftermarket and Modability-- Stinger. No contest here. So many choices I've seen in a month. GSF is just too rare.
Overall Quality--GSF. Of course I have to say GSF here. 10 years old and 145k miles and still going. The car is so reliable and well built. Not a rattle in the GSF at its age. I can only hope I get the same reliability out of the Stinger. Already had to fix the hatch rattle and wrap the window trim in the Stinger.
The GSF is a better drivers car. The Stinger is a better daily drivers car. Remember I'm comparing an 80k msrp to a 54k msrp. Price for smiles the Stinger wins so far.