Both sentences are grammatically perfect. It is, however, the usual thing
"It's very hot today."
In this sentence, today is an adverb.
In your other sentence, "Today
is very hot," today is a noun and
is the subject of the sentence. You might use this sentence in
a conversation like this:
Person A: The weather has been so hot lately.
Particularly yesterday. Yesterday was very hot.
Person B: No, today is
very hot. The temperature is at a record high today.
This kind of conversation
is not very likely, but it is possible.
So, in answer to your question ?for
most conversations, it's appropriate to say:
"It is / It's very hot today."