If your aim is to be in government, it's a different calculus than private.
Private firms seem to be more skeptical of policy roles, even in relevant subject matter. They prefer someone with an "interest" in the subject matter but who's picked up hard legal skills - even in a different subject.
If the policy position is in the same field, it would seem to be relevant experience and not just a "filler" position. I have no relative experience for this question, but would imagine the transfer from policy back to law would become more problematic the longer you held the policy position, but shouldn't be too problematic early on (and "I took an available position in my area, but am interested in moving back into law" is a decent sales pitch if you take a later job interview).
For example, if you did the policy job for 1 year, then went to move back to a firm, your "lost time" is less as you've hopefully gotten the relevant exposure to the law, but aren't senior enough that there's an issue in learning to be an associate (ie. handling grunt work). It may even be beneficial experience as it adds a different prospective to the problems. However, 5 years of policy before moving back? Might be a different story, as you are more closely aligned with a 2-3yr associate in realm of associate tasks you are familiar with, but probably don't want to be treated like an intro associate task-wise (all speculating, depends entirely on both policy experience and the tasks your transfer back into).
In short, you're not forever excluded from looking for openings while holding the policy job, so if it's of potential interest this may be a "FT is better than contract" conclusion.
And who knows, maybe you love the policy job?