Greyhawk was really the first campaign world (primarily a carry over from Chainmail and the original group) and then TSR soon to be Wizards began with the Forgotten realms (A creation of the great sage himself, many of the details of its creation available in editorials in dragon) over intelectual rights issues and no new content was created for Greyhawk. This would put both worlds at approximately the same age. Along the same lines all the literature was done in the forgotten realms and Krynn (*sp). Wizards eventually made amends, I remember reading in Dragon somewhere that Gary was brought back on as a creative consultant (one of their editorials) and low and behold they started re-releasing old modules from Greyhawk and new ones in development as well as novels. The only shortchange Greyhawk really has is that theres very little substance to the world whereas the realms has entire histories that have been explored in games, novels, fansites, fanfiction, etc.
Both of these worlds adhere strictly to the precept of High Fantasy, part of what makes them so popular. As far as levels of magic, Tome of Magic in its appendix made several outstanding points about how prevalent certain levels of magic should be, and every shop keeper carrying a +7 broadsword of lifestealing is a bit... odd to say the least.
(this is how I recall it, and was done w/o any current fact checking)
