"Sign Up/Join Now" versus "Learn More/Find Out More"

Does anyone have any insight regarding the use of a Sign Up "Call to Action" (CTA) versus a Learn More CTA? Here's my thinking:

"Sign Up" is ideal in situations where the lead is MOST informed about, or has the MOST confidence in a product. (As much as possible, anyway.) Either the product is decently well-known (it's already popular or the lead has been referred by someone else) or the CTA follows a flow where a lead's been fed a series of strong value props.

"Learn More" is ideal in situations where a lead only has very limited knowledge of a product, ie., in the case of a banner ad where you only have 20 or fewer words to encourage a lead's interest or curiosity. In this case, "Learn More" feels less risky to a user who might not be ready to sign up, therefore increasing conversions/clicks.

Thoughts? Any research out there on this?