If you're talking E39's, they were not sold at the same time. The 525i was introduced in '01 when the 528i was upgraded to 530i.

You could get most options available on the 528i/530i on the 525i also. However, I've read that one of the negatives on the 525i is that the exhaust manifold comes as one piece with the cat converter and it is mucho $$$$ to replace it.
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HarryPR
BMWCCA #19290