Is there a rule on which is correct in a given usage?
For example, which (if either) of the following is more correct:
I’ve spent a lot of money on my hobby over the years.
I’ve spent lots of money on my hobby over the years.
I feel like there should be some distinction between the plural and the singular but both seem interchangeable.
I can see no difference at all grammatically, but the first example seems to scan slightly better.
The only difference I can see anyone agreeing on is that “lots of” is more informal. In theory, “lots of” is more than “a lot of,” as there are multiple theoretic “lots.” In practice, though, they are interchangeable. Except I think “lots” often carries a bit more emphasis on the large number.
At least, that’s the feel I get in your example. I can’t even read it without putting a slight bit of emphasis on “lots of” that I don’t put on “lot of.” (Technically, I say “lotta” and “lotsa.”)
A ‘lot’ is a collection of things. For example, one might go to an auction and buy one lot of knick-knacks. Or they may buy two lots or three lots or whatever.
So when I hear ‘a lot’, I think of ‘much’ or ‘many’ of the thing being mentioned. When I hear ‘lots’, I think of many lots of many things. i.e., One lot vs. multiple lots.
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In colloquial English they are interchangeable. Neither is truly acceptable for formal English, so it doesn’t matter there.
I did find a post on the on the English Language and Usage Stack Exchange (ELU) that illustrates a difference between “lots” and “a lot.”
Rather then quote the entire post, I’ll summarize: there are times when “lots” refers to frequency while “a lot” refers to quantity. The example is “I’ve eaten lots today” vs “I’ve eaten a lot today.” The first is only frequency, while the second can also refer to quantity.
The same post also indicates that there is a construction “That’s a lot of” that cannot be substituted with “That’s lots of.” The example given is “that’s a lot of water,” compared to “that’s lots of water,” which I agree sounds completely unnatural, and would only work in contrived situations.
And, yes, I’m aware my “summary” is longer than the post in question. Consider it an explanation instead, as I think I explained it better.
It’s like flammable and inflammable. Pick whatever suits you.
The first could easily refer to quantity.
It doesn’t matter if someone can construct a contrived situation. Real speakers don’t distinguish.