Look for differences in price, stability and redox potential. Are we talking in vivo or in vitro? For in vivo use, you'd have to take metabolism also into account.

Engelbert Buxbaum. Thanks for the reply. It is for in vitro, and cysteine is almost half price of NAC. You're definitely right for the metabolism issue.

Here's an article comparing, amongst other, NAC and cysteine as antioxidants in vivo:

https://www.cell.com/cell-chemical-biology/pdfExtended/S2451-9456(18)30033-3

To reduce disulphides in vitro, Dithiothreitol is usually preferred to Cysteine, since it is able to reduce both cysteines of a disulphide bond, making it less likely to end up with a mixed disulphide intermediate.

https://en.wikipedia.org/wiki/Dithiothreitol

Thank you Annemarie.

Yes, it can. I use solution containing cysteine, urea and NaOH (up to pH 10) to reduce disulphide bonds in keratin-rich material in order to extract the soluble protein

Antoni Taraszkiewicz Thank you Antoni. How long would such a solution be stable at 4 degrees?