Both are correct.

It is much more common to hear “it’s very nice today” and less common to hear “it’s so nice today” I think we actually use phrases like “it’s a beautiful day today!” or “The weather is perfect today” or even “its a perfect day!” to describe the same idea. Any of the above options can be used and understood, but the ones I provided above are more colloquial in English.

Both are correct. You can say either one. Good luck!