Which is better for the Nurburgring in R class?
I’ve used both extensively and I think they’re as good, depending on your skills and/or preferences.
I’m using exclusively the F1 GT at the moment because it seems to have more grip in corners. Also made it to #7 in APAC 
The McClaren F1 for me. I put in 30 or so rivals laps at Nordschleife with the GMA t50 to get a top 80ish time. Then I beat my PB on my 4th lap with the McClaren for 34th. Some cars just feel right for some tracks, and the McClaren feels right for Nordschleife.
Are you using the no aero build for the T50? It’s ridiculously good and very easy to drive. So far as I can tell the T50 and F1GT are very similar pace.