Having driven the 2018 Mustang GT with performance package recently, it's a lot of car for the money. Power is incredible and handling surpassed my expectations.
I think the G70 will be a very different experience with appreciably more refinement in exchange for decidedly less muscle.
A G70 will be considerably more civilized car than a Camaro or Mustang, with a much quieter exhaust and a more elegant interior. The Mustang interiors have certainly improved but still lack the details and amenities of the G70. On the flip side, the G70 is a powerful car that promises to perform well. But the Mustang GT, with 95 more horsepower (if we believe the manufacturers) is in a different league entirely.
For context, I've owned several sports cars, including a 240Z, three RX-7s (the last a twin-turbo), and a BMW Z3 with the 3.0 liter engine and handling package. My wife has had a few sports sedans, including an Acura Legend and a 3-series BMW. My last couple of cars have been Infinitis, first a 330 HP G37 with 6-peed manual and currently a 328 HP QX50. The QX50 is the first primary-drive car with an automatic, going all the way back to our first car in 1969 but all subsequent cars will likely be auto for me because it enables my wife to drive it more easily and, frankly, automatics are a lot better than they used to be.
If I decide to get a G70, I'll likely trad my wife's V6 Outback and she'd take over the QX50, which only has about 26,000 miles on it.
Whether I decide to get a G70 depends on several things, the two most important of which are, first, how much I like it when I actually drive it and second, how much they want for the car. I want something that is decidedly more fun to drive than the QX50, which is a very capable car and has good power but it's hard to forget that it's a crossover and not something with mre of a performance-oriented personality. The Infiniti G37, in its day (2009) was a coupe that had a nice sporty feel to it, with good power and handling for its segment. I want something more akin to that but updated to current standards of performance and technology. The closest thing from Infinit is the Q60 Red Sport 400 coupe, which checks a lot of boxes but is a bit outdated now. It's certainly still a fall-back (in either coupe of Q50 RS 400 sedan version) if the G70 turns out not to be as good a value as anticipated.
What sort of driving do I do? I don't race but I do drive aggressively when I can do so safely. And North Carolina still has plenty of twisty back roads that are conducive to the enjoyment of a car that is powerful and handles well. I don't have a daily commute, so living in traffic isn't a consideration (something that allowed me to consider the Mustang). So, something that doesn't feel too much like a living room is decidedly an advantage from my perspective. I much prefer a firmer ride and more responsive engine to a "comfort balanced" car. I do occasionally take long trips of several hundred miles each way and I want a car that won't punish you on those (the suspension on the '93 RX-7 twin turbo got pretty rough on the backside after a hundred miles or so) but that still feels like I'm driving it instead of it driving me.
All things considered, the G70 is potentially a choice that will suit my needs. But until I drive it and know what it will cost, I obviously can't make a final decision. And I'm not so wedded to the idea of a G70 above all other alternatives that I'd feel crushed if I felt I needed to walk away from it, either because of it's not meeting my expectations as a driver or its costing more than I think it should. Of those two deal breakers, I think the latter is the one that I'm more concerned about. I believe it is a good car, from what our Canadian friends have described. Whether it's an excellent value as a car aspiring to the luxury segment but with a thus far unproven record, remains to be seen. Part of that is also a judgment about the long-term viability of the Genesis brand in the US. If Genesis blows it with the G70 (by overpricing it, for example) I think they'll have a tough time surviving in the segment they're targeting, given the wealth of alternatives from more established and highly respected companies. I hope they get it right and remain optimistic that they will. But only time will tell at this point.
Hope these observations are useful to you as you consider your next car.