Well, I'm glad to hear you say that. I actually read you as leaning the other way. I'm glad I was wrong. So let me jump in on this side.
The question is not opportunity vs. self-validation, it's opportunity vs opportunity. If you believe in what your doing, and you can keep chugging along doing that, that's what I would do. Thank the company for the opportunity but tell them you are committed to your startup, they will respect that. And if they don't, you probably didn't want to work there anyway.
On the note of experience. At the job you are being offered, most likely you would gain specific technical experience. But you get hordes of experience from running a startup too. The startup experience tends to be on more of a macro level and you will find it useful in many aspects of your life. Running your own company will force you to put on lots of hats and give you a more complete look at how things fit together on a grander scale.
Not saying that the job experience is not valuable. Just reminding you not to forget about the experience you are already getting at your startup.
Self validation is cool too.
So what's this startup? =)