Teebin I am no sure who's actually saying this, but they are completely wrong. I've ridden Gringott's now 6 times and I can say without the slightest shadow of doubt that the front car front row is the best for the illusion/immersiveness. There's really no contest here. You're either getting a gorgeous completely unobstructed view of everything (front car) or you're in the back car where you see another car in front of you, seeing the tops of people's heads, and even a pole sticking high up from the back of that car that is right smack in the field of view of one of the main screens.
Again I'm not disagreeing that the second car gives better thrills, but you said "illusion" and there's no way it gives a better illusion than the front car.