>>1355792Gelbooru pulls every new image from Danbooru, but like has been said, Danbooru has stricter "standards". And more than a few bitchy uptight mods/janitors.
Example: a search for "huge_breasts" nets you quite a bit of /d/ on Gelbooru, but near none on Danbooru.
A lot of people just use Danbooru because it was the "first", like 4chan and Rapidshare. But new sites have surpassed them in many ways.