I would agree with John that a set of naturally matured stones is preferable, assuming they run consistantly with each other and aren't worn out!

It's really tough to come up with a recommendation without seeing the stones themselves. What type of granite is it? How badly pitted are the stones, what's the running surface width. Also, more importantly how well are they matched, what is their speed and how much do they curl with your normal setup?

That being said, there's nothing wrong with inserted rocks. All the major championships use inserted rocks and they're fine there. However, with any new set, or inserted set you will need to work on your rocks to keep the curl in them over time. Fred Veale is the guy to ask about that. But, you won't hurt your rocks by getting them inserted.