I race solely in VR, my rig doesn't even have monitor, but I fix things in GTR2 on 21x9 and it seems to render correctly. IMHO, the wider the screen the more true to life racing is. Human FOV is 220degrees, FIA helmet has 180deg FOV. Even when you take drivers license test they check if you see well at edges of your vision. So my vote would be 21:9 and if you can I would even go triple setup. One drawback though 21:9 will be a bit more demanding on GPU/CPU but not by much.
Jempy mentioned menu issues, but there's a fix for Steam GTR2.exe that lets you have better UI scaling on triples or ultrawide.