Sounds like a great opportunity to test your potential employer. Tell them that you must do a demo and ask for advice. If they will say "OK, do the demo first"

Sounds like a great opportunity to test your potential employer. Tell them that you must do a demo and ask for advice. If they will say "OK, do the demo first" then just do the demo and take the offer. But if they will insist on the interview on the demo day....then you should think twice, because it means they do not appreciate employee loyalty. A bad sign. Nevertheless, good luck!