I'm currently in the (a) situation and could see myself here for the rest of my working life. Well, the "stable" thing could go away at any moment, which is really at the crux of the matter.

No job is truly stable. And if I'm not doing anything particularly exciting, it means that I'm not learning new things. So when this job does go away (and I'm pretty sure it's a "when" rather than an "if"), then I'm sort of behind the curve. So that's an issue.

Interestingly, my friend Tod was in situation (b) recently and got out of it to come work where I work. It was his experience in some of the new stuff he'd learned with (b) that allowed him to get the job at (a).

So it's all trade-offs. I'd say if you're ready for excitment and change, go (b). If that scares you, stick with (a).