(GPLv3 isn't anywhere near as popular as v2 was)

What is the difference, if I release something with  GPL version 2 or 3?

Do not quote me on this, because I'm not sure, but I recall reading that the main difference between the (L)GPL v2 and v3 is that v3 allows the library to link to and use other libraries that may be using licenses different from the (L)GPL. It arose from some ambiguous wording in v2, so they modified v2 to clarify their intention. I think they clarified some other ambiguities as well. It's essentially the same, but more explicit in some of the previously ambiguous areas. This is just what I remember reading, and it could entirely wrong.

(GPLv3 isn't anywhere near as popular as v2 was)

What is the difference, if I release something with  GPL version 2 or 3?

Do not quote me on this, because I'm not sure, but I recall reading that the main difference between the (L)GPL v2 and v3 is that v3 allows the library to link to and use other libraries that may be using licenses different from the (L)GPL. It arose from some ambiguous wording in v2, so they modified v2 to clarify their intention. This is just what I remember reading, and it could entirely wrong.