Both sentences are grammatically perfect. It is, however, the usual thing
to say:
"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."