I've been using Protoform as it's easier to find. I never tried other brand. I now its very boring when you arrive in the track everybody is using Stratus 3.1
I tried a Mazda 6 of a friend and I felt the car much better compared with Stratus 3.1... gains in fast corners S type and tight, but i loose speed on arc type corners
I saw some guys using Parma M and it's seems to be worst in material quality but the design is very interesting