I think it depends on the market more than the domain itself.
If the singular is already a well-known brand in the same industry, I'd be cautious. You're likely to deal with user confusion, mistyped emails, and accidental traffic leakage. That's a branding challenge that can be difficult to overcome.
On the other hand, if the plural is the more natural way people refer to the product or service (for example, "Servers" rather than "Server" in some contexts), it can work perfectly well provided there isn't a direct conflict with an established competitor.
Personally, I'd evaluate the branding as a whole rather than making the decision based solely on singular vs. plural. Sometimes a different, more distinctive name is the better long-term investment than settling for a variation that's constantly compared to someone else's brand.