I voted for in-car, but I as others have said have ALWAYS used bumper cam in GT4 and still find it faster in GT5P.
BUT, I love the in-car, and use it as often as I can! (Read Daytona, Suzuka, London)
The only thing I find (again, as others have said) is that the view is slightly restricted in in-car, and IMHO would benefit from some corner-look (if that's the right phrase?)
i.e. As you turn the wheel, the view moves as if you were looking through the corner at the apex/exit. As an experienced bike rider (and track-day fan) I can say with certainty that when going into/round/out-of a corner I am never looking straight ahead, I am looking where the bike/vehicle is going.
Just my honest opinion.
Neil