I think Motorsports headers are good for the money.
They have the all important thick flange needed so they don`t warp. They seem to last a good few years.
Now, for the real money(approx $400.00)you get a header with equal length tubes, choices of tube diameter. What may be refered to as a "race header".
There is a bit better quaility in them as well (IMO).
I would have to say that these features make a difference, but I don`t have proof on paper.