NFT with a word is quite uninteresting. Of course, you may catch a sale if its strong enough, if you own that thin number of good words (NFT Market, Compare, Swap, Art, Broker etc), but in reality, a brand named "Kittens" or "Pluto" has a better chance of being the next hot NFT brand, completely outside of the term NFT, considering very few major brands actually have NFT in it, IMO
Singular form => more brandable + a bit more popular term
Plural form => applies better for a marketplace than other projects, it gives better the "quantity" meaning. Some people (likeme) feel better adding the "s" when talk about quantity.
Both terms are great, in my opinion their selection relies on the direction a company has.
When quantity must be highlighted, then plural>singular. No matter how many letters it has. The most important thing is what the buyer have in his mind to do with the domain.
NFT (singular) is the stronger variant on paper, but truth is the plural (NFTS) being a $15m domain asset and one of the largest confirmed domain sales of all time, does it really matter?
$15m is life changing, if one owner owned the domain previously, this person's children/children should never have any money worries and they are now richer than many celebrities.
For Marketplace, NFTs I think is better, because it offers alot of NFTs. But for projects like the ones introduce Crypto Punk or Bored Ape Yatch Club, NFT is better.
So both are good, but what is the purpose of the domains, every domain is unique