Way back in the day I spent a lot of time fiddling with the FFB parameters in F1 Challenge (the game that Simbin first modded to create the original GT Mod which evolved into GTR and GTR2) and posting my settings to the old Racesimcentral forums. In fact I even got a thankyou in the GTR2 credits. I also was lucky enough to get into the alpha and beta test programs for iRacing. I subscribed to iRacing for a couple years after the beta ended as well. It really is a fantastic sim. Now though, GT5 is my game - but thats really more a matter of circumstance and convenience than anything to do with which is more realistic.
There are different ways to approach these things and they can all end up with a decent result. I'd suggest GT5 is more of a "look-up table" sort of a physics engine - data is entered and then things are tweaked and adjusted until there is the impression of a convincing driving experience. Iracing otoh really operates on an engine that has far more actual physics going on under the hood. The tire modelling (which is really a crucial part of a car sim since the tires are the only thing in contact with the road) is just hugely more complex in iRacing than it is in GT5 and its constantly being improved. And the tracks in iRacing are just unbelievably good. As good as GT5's version of the Ring is, if iRacing ever did a scanned version I'd have to subscribe again just to drive it becuase it would be amazing. With a properly set up FFB wheel, iRacing is just incredibly convincing. You really can feel the "rubber on the road" so to speak. GT5's FFB feels really leaden and ham-fisted by comparison.
But GT5 is a beautiful game and it succeeds marvelously at what it wants to do - give you the feeling of driving an incredible array of different cars as well as the fun of acquiring and collecting them. While I wouldn't say its super realistic, its definitely fun and the driving model has enough subtlety that you can run hundreds of laps and never feel like you can't still improve your times.
There are different ways to approach these things and they can all end up with a decent result. I'd suggest GT5 is more of a "look-up table" sort of a physics engine - data is entered and then things are tweaked and adjusted until there is the impression of a convincing driving experience. Iracing otoh really operates on an engine that has far more actual physics going on under the hood. The tire modelling (which is really a crucial part of a car sim since the tires are the only thing in contact with the road) is just hugely more complex in iRacing than it is in GT5 and its constantly being improved. And the tracks in iRacing are just unbelievably good. As good as GT5's version of the Ring is, if iRacing ever did a scanned version I'd have to subscribe again just to drive it becuase it would be amazing. With a properly set up FFB wheel, iRacing is just incredibly convincing. You really can feel the "rubber on the road" so to speak. GT5's FFB feels really leaden and ham-fisted by comparison.
But GT5 is a beautiful game and it succeeds marvelously at what it wants to do - give you the feeling of driving an incredible array of different cars as well as the fun of acquiring and collecting them. While I wouldn't say its super realistic, its definitely fun and the driving model has enough subtlety that you can run hundreds of laps and never feel like you can't still improve your times.