I'm currently doing an internship at a company for a fixed-month period, which I'm now close to the end of. Recently they approached me and asked if I would like to stay longer, instead of going back to university once my period was over.
Originally I was planning on going right back to school, but I've since received an offer from another company that I can't refuse. The offer's great, but the company itself is somewhat of a better fit for me as well. (To be clear, it's just another internship, I'll be returning to my degree to finish it after that final internship.)
How do I politely decline the first company's offer? Do I mention the fact I'm leaving for another company? What should I say?