It depends how much you like the subject/place of the first one. It is a bird in the hand decision, as you may not be offered the second, and lose the first if you dont accept.
If the one you have been offered sounds interesting and you like the people you should accept it and cancel the second interview. If you don't like the place or the people, then tell them you will decide after the second interview. They may say that the offer does not stand that long, and you must be prepared for that.