70-200 f4 or 70-200 f2.8

I am shooting (American) football with a D300 and 70-300 VR lens.  Look at my 9-page gallery.  http://www.titansnapshots.com/Games/Wheaton-College-2012/24515362_37ZQG8#!i=2165231439&k=SjKtczb and about page 9 the sun sets.  You can see a loss of color.  Our football season ended 3 days ago, so I am now trying to sell my D300 and put the money toward a newer body with higher ISO capability in preparation for next year.  I would like a faster lens, but I don't think it will help as much as having a faster sensor.