Here is something many people don’t think about…bleed-through from the verso when you do a scan.
You can put a black backer on the flatbed scan, but it darkens the whites. You can improve on the dingy whites with post processing. (PP) The sheet fed scanner, when the light is on to scan both sides at once, minimizes bleed-through a good deal. But the blacks are not as deep and the scan needs PP to compare to the flat-bed scan. The sheet fed scan is also not as good in terms of resolution to the flatbed scan. Although for 98% of the time the sheet-fed output is fine.
I’m going to show you some scans to illustrate bleed-through and all the rest of what I have mentioned above. Other than cropping, no post processing has been done on the scans except for the sheet fed scan labeled as post processed.

Scan from flatbed scanner with white backer. You can see some bleed-through in the white background near the top.

Scan from flatbed scanner with black backer. You see less bleed-through in the white background. But whites are dingy compared to the white backer flatbed scan.

Scan from sheet fed scanner operating on both sides scan mode. Less bleed-through, but blacks and overall contrast is not as good as flatbed scanner.

Same sheet fed image as above with post processing.

The verso that is bleeding through the front of the image.
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