“Thank you very much” vs. “Thank you so much”

Some people used to say:

Thank you very much.

Where others say:

Thank you so much.

Could anybody please explain what differences there may be between those, whether of correctness or usefulness or anything else that separates one version from the other.

I wouldn't agree that either is necessarily "more genuine" as @JohnPeyton has suggested. Intonation and emphasis can affect the intended meaning as much as the actual words used.

Additionally, I think usage will differ between different English-speaking regions and countries, with some nationalities being naturally more reserved, and others naturally more 'gushing'. Thus some will tend to use superlatives much more readily than others, with the result that the former may have to use 'super-superlatives' to express greater thanks!

In summary, and answering OP's direct question:

• Neither is wrong and hence neither 'more correct'.
• Both are equally useful, as is just plain "Thank you", "Thanks", "Many thanks", and other variations.
• I do agree with @JohnPeyton that "Thank you very much" is a little more formal, and "Thank you so much" is less formal.

‘Thank you so much’ often seems more genuine. It happens to be less formal, and it implies that the person saying it really means it.

‘Thank you very much’ is often used out of courtesy, or to make a ‘thank you’ a bit longer. Sometimes it’s even used sarcastically. Of course, it can also mean what it says, but it’s barely stronger than a simple ‘thank you’—just longer.

Thank you so much is becoming way over used and doesn’t come off as genuine anymore. It’s almost as if everyone were hypnotized into saying Thank you so much instead of the good old Thanks a lot.

Both of them are grammatically fine.

Having said that, I recently returned to the USA after living abroad for 25 years, and I noticed people using (or overusing) the phrase Thank you so much rather than Thank you very much or Thanks a lot, both of which I remembered being standard things that people used to say.

Saying Thank you so much seems to have become a cliché, and I personally refuse to say it. I am not one to follow verbal fads.

If you dig into your memory banks you will realize that "thank you so much" only became widely used in the last couple of years. Listen to people on TV and you'll now hear "so much" almost exclusively. It is just a pop culture trend. People hear it then repeat it. It is no more sincere than "very much." Watch clips of morning news shows for example,from 5 years ago or longer and you won't hear "thank you so much" at all. It is part of an overall fascination with "so." For example, "I am SO going to the movies." There are no degrees of going to the movies. You are either going or you are not. So is so being so overused now.