My opinion, between the F-GT and the GT Ultimate, I would go for the GT Ultimate because I prefer GT racing over Formula racing. Now I don't have a rig, and I am looking to get a wheel stand similar to the front half of the GT Ultimate.
The GT Track is supposed to be far more rigid than the GT Ultimate, so if you are considering at stronger wheelbases in the near future you might want to spring for the Track.
If set on a NLR cockpit, I would suggest (as the post above) going for the GT Track. I have one, with NLR V3 motion, a DD wheel and shifter mounted to it and is very sturdy. Have even modded the pedal deck with some 8020 (to raise the height) and is super solid.
There is very very slight wheel deck wobble (due to the DD wheel) but you will not notice it during racing, and if you have a belt wheel then I’m sure it’s rock solid.
Just a word of caution, if you are looking to replace the seat with another aftermarket one, then you will need to do some drilling as the NLR GT Track seat mount does not follow the standard hole pattern.
Would also suggest you have a look at building an 8020 rig - not much difference in price and much more future proof and won’t move a mm even during the toughest workout