In my limited knowledge GPLv3 is better then GPLv2. Because big parts of code has been rewritten from the beginning or are new this allow to make them GPLv3. I think it is possible in Shareaza licence to point that this and this part are GPLv3 and this and this GPLv2 still, or just mention that parts are 3 and parts are 2. And with new version make more and more GPLv3.
This also relates to PeerProject which could be I think full GPLv3, or even ought to be.
I dont know if there is any newer like GPLv4 licence.