I'd be using it for mainly logic studio, along with the common tasks of mail, ichat, safari, itunes, etc.
I'd say you'd be better off saving the $1600 and going with the 2.8GHz. The performance increase isn't that significant to warrant the high price. (you could buy a mid range macbook with the extra money instead).
Barefeats' Rob points out the 3.2 will have a much higher resale value. I'm getting the 3.2 with 32GB of RAM and four 1TB HDs in a Raid 5 because I want maximum performance. I can't put a price on that any more. But the 2.8 is still very fast.
Look at Barefeats comparisons to help you decide. They have the 3.2 and a 2.8 comparison across a wide variety of applications and games.
Barefeats' Rob points out the 3.2 will have a much higher resale value.
The question is how much higher is the resale value.