"It is really hot here."
Translation:Det er kjempevarmt her.
So would it just be personal preference? Are there regions that favour one over the other?
It would mostly come down to personal preference and the degree of formality called for. The prefix "kjempe-" is somewhat less formal than "veldig", and more common in spoken than written language.
Regional differences are always a factor, but then the choice wouldn't just be between "kjempevarmt" and "veldig varmt" any more.
The use of kjempe- (kjempevarmt, kjempebillig, kjempegøy and on and on...) is more usual in spoken than written Norwegian. Plus elderly people don't use it much (anyone aged 70 or over would normally prefer other words).
there is heit but it is not used much as far as I know. Usually context will help you understand whether someone means warm or hot.