Take the job and learn what goes on in a big company. Off course at the same time start working on your business idea as well.
Benefits of working for a company
I'd go work on my startup because thats what I want to do. What do you expect people to say? Do what you want to do.
You will notice that 'n' is not a very large number. But, if you spend more than  during your 'n' months, then realize that it will be very very hard to come out of it. (you have the flawed entrepreneurial gene and if you can't resist the temptation you might be dooming yourself to torture).
From my experience - before launching a business, a person would need to know a lot of things/skills/experience and many friends/contacts.
So if you're starting at an early age (before 20s), just jump into business. Over the time you'll gain experience and make contacts.
Otherwise, take up at least 3 jobs, spending at least 6 months in each before jumping into biz. That's a smart approach, less risky and you'll have much clearer picture.
That was exactly my situation, I rejected job offers, "continued" with my venture. However, 8 months later I took up one start-up job offer. Learned many new things there and this helped me a lot in my business.