I have both ISF and GSF, plan on keeping both as they are incredibly difficult to source here in Australia considering the situation.
I have had the GSF for 3 years now, absolutely fantastic car, a pure sleeper and fun car to be in. Like others have said the TVD really transforms the GSF and gives you more confidence vs the ISF which seems more RAW. I have only recently acquired the ISF, it has plenty of miles on it but feels solid.
My ISF has headers and feels as quick as the GSF in a straight line, although I am planning on doing some track work with both cars later this year. I am interested to see my times on both cars, I am certain they will even.
GSF is definitely more fuel efficient considering it runs in Atkinson cycle whilst cruising, ISF drinks more juice for sure.
If you have kids, then I would highly suggest to source the GSf as its very comfortable ride. ISF does feel firmer than the GSF.