Other than that I'm not entirely certain about the differences between the two archives, but I can point you to a thread we had shortly before you arrived on the list about the same subject.
http://lists.debian.org/debian-amd64/2004/10/msg00260.html
Stefan Lüthje wrote:
Hello,
I read this list since last week. But my question is, which tree I should use. At the moment I have the /pure64. Make it sense to use the /gcc-3.4? If yes, what is the easiest way, to change the distribution.
Best Regards
Stefan Luethje