I had to make a very similar decision about a year ago. I came to the same conclusion: they could count on me just as much as a freelancer than as an employee, and I'd rather keep my options open rather than get tied to a single company, because I have my own, unrelated plans in the works.

I suppose one could make the argument that I'm going to miss out on a huge pay-day if I'm not that sort of committed. I am balancing the likelihood of that occurring + the likelihood they'd expect unreasonable overtime from me versus the likelihood that I'd find that my own way and the likelihood I'd be fine working overtime for myself. Freelancing is the only way I know how to maintain my work-life balance.