It really depends on your goals.

Assuming the previews provide enough information to convince people to sign up, your conversion rate would likely increase if you require sign up.

Long term though, it might have some drawbacks. Google would have less content to index, so you might not get as much search traffic. Fewer people might end up sharing the pages, because they might see only the preview, not bother to signup, and not share the page.

At the early stage I would lean towards making it public. It increases your "luck surface area" as more people will see your content, it likely will get shared more frequently, etc. Have a look at @harrydry's marketingexamples.com on how you can still build a mailing list while providing the content publicly. (Both his own site, and some of his case studies are worth a look.)