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Are you lowered at all? If your lowered more than the stock bolts in back and have cut the bushings, I think people are saying you'll need the Bilsteins due to piston length. Looking forward to hearing other replies.
I had the same decision to make earlier this year... went with the Bilstiens b/c my car is lowered and the Bilstien has a shorter shock body. A stock shock on a lowered car will be a bit compressed causing the shock to wear a bit faster.
The Bilstiens supposedly have better rebound control than the '04's but I have no way of verifying this. I can say that the improvement over my '01 shocks w/ 45k was very noticeable. The ride is a bit harsher, but the car handles dips awesomely.
I have run them both in the rear and I feel that the Bilstein sports have too much rebound for choppy pavement, nice though for smooth fast sweepers. The 04 ZO6s are very nice on choppy tight turns, very little side stepping, the Bilsteins sports were a little scary with the amount of side step. I did try Bilstein heavy dutys and they were a nice compromise. The 04 Z06 are the most confidence inspiring, the Bilstein heavy dutys provide the most comfortable ride.