I think most people looking at buying a G70 (especially the 3.3t) are looking at the vehicle because of what you get for the price. Obviously there are many similarities and differences between the Stinger and G70, butwe are looking at bang for your buck at the end of the day. I'd argue that anyone shopping with regards to that "bang for your buck" principle would be at least looking at the Stinger and/or considering its value.
With that said, I still haven't been able to test drive the G70 (I live in the LA/OC area ), so I can only speak to my experience driving the Stinger GT2. Found it fairly impressive and once you consider the $61-65k MSRP for the S4 and 440i I got to test drive, even more so. I'd expect to find a similar experience in the G70, just at $10k higher
I've test driven both the G70 and the Stinger GT. While some things are similar, the overall driving experience was quite the contrast for me. Interior features aside, I felt the G70 to be significantly more responsive to steering inputs, I could feel the road surface WAY better, and it just felt more athletic and sporty... This may be an exaggeration, but I kind of felt like the Stinger seemed really numb and boat-like in comparison! The powerband on the 3.3, on the other hand, is pretty similar.
Of course take my observations with a grain of salt; but, I think it's worthwhile to test drive them both and see what you value most! The deals on stingers right now aren't easy to ignore.