In my opinion, the 2015 5.0 ultimate would be a good choice due to the features and affordability. The 2015 5.0 Ultimate has most of the tech as the newer model G80 and nearly the same styling. You really cannot tell the cars(2015 or 2018 models) apart on the road unless you know the subtle details like the LED DRL or the
badges.
The 2015 5.0 Ultimate also gives you the sport package with adaptive suspension and the 5.0 V8 engine as well as the luxury interior package with the real wood trim. Both of those features has been separated in 2017+ G80 models where the G80 3.3T sport only gets the adaptive suspension which upsets me because I love the adaptive suspension and 5.0 V8 engine.
Yes, some will argue the 3.3T engine is quicker when upgraded; but not in stock form. Plus, I believe that many people in the G80 buyers market do not want their large luxury car engine modded out due to possible reliability issues when boosting the turbo engine past factory specs. The 5.0 engine is powerful and reliable in stock form.
If you can live with driving an used older model, than I believe that the 2015 5.0 Ultimate model is the best overall deal that can had for about $25000 or less.