McLaren F1 GTR clearly shows its age when it competes among the more modern Gr.3 machinery. Sure, it's a monster on the straights, but once it sees a corner, it feels like the suspension has arthritis.
The NSX, on the other hand, is a better performer overall. On Daily Races (on the Asian region), it's a common sight on Top 10 fastest lap leaderboards. It is twitchy, but it has a way better tire wear than the F1 GTR.
In short, get the NSX. Alternatively, the R8 LMS.