There are lots of posts about differences between G&K and vanilla, most of them dating to when the expansion came out; you can search and find them. I think G&K is a big improvement, and haven't gone back to vanilla since I got G&K. One of the biggest changes with religion is that you can use it to overcome many of the happiness issues, and thus can expand to bigger empires than you could with vanilla. Since there are also rumors about a new expansion coming "real soon now", however, you might want to wait for that before contemplating an upgrade.