The old one was originally $3200, but they dropped the price on it until they are sold out. Obviously, they can't sell the old model for $3200 any longer.
The new one is a minor upgrade to make it compatible with the Sigma USB dock, which may finally be available this week. It allows you to make AF adjustments to the lens to match a camera. This might be handy if you have a camera without AFMA, but if you use multiple cameras, you can only adjust it for one.
I'm waiting for tests of the updated model, but don't expect to see many, since its supposedly optically the same. There are plenty of tests for the older version around. Personally, I snagged a Canon 70-200mm f/2.8L MK II from the Canon refurb store for $1700. for me, getting the extra 300mm is not worth another $900
If you need 300mm at f/2.8, the $2500 versions won't last long, so save $1000 and grab one.