The IBM x3550 Hard Drive Accessories Website shows that both the 43W7598 and the 39M4526 marketing part numbers have the same replacement part number of 39M4529 (see screenshot below). This appears to confirm that the drives are functionally the same and only differ from a marketing offering standpoint.
The only difference that I could find was in the type of warranty service:
- Express model 43W7598 = Limited Warranty
- non-Express model 39M4526 = Customer Replaceable Unit
To find this information, I went to IBM's Storage Accessories & Upgrades Shop and then added one of each into the cart. At the next screen, I clicked on the product description for each to get the screens shown below. Going this route, both drives were priced at $139.00 each.
Based on the IBM Express Advantage portfolio webpage, it appears to me that the Express designation is just a marketing offering geared toward the midsized market. I wouldn't be surprised that they are offering the same products under different model numbers for different prices/markets.