Hiya.
I like both, for different reasons.
I like Greyhawk because I've been DM'ing in it since the folio was brand spanking new (re: 1980, iirc). I find Greyhawk to be the defacto setting for "classical medieval fantasy".
What I mean by that is that when I picture a group of guardsmen lead by a knight and a ranger, out patrolling the borderlands of the Yeomanry and The Tors (north of the Hold of the Sea Princes), I picture men on horses. They are outfitted in ring-mail, have spears, perhaps a mace or warhammer at their side, and a round, wood-and-metal-rimmed shield. The knight has chainmail, a lance, a longsword, dagger and a steel shield emplazoned with the standard of the Yeomanry and his house. The ranger is in studded leather, and has a battle axe strapped to his back, and a short bow and quiver of arrows, along with a dagger. All are regular men, of average, if a bit more muscular, build of the typical human male. Equipment is mundane, with nothing out of the ordenary.
Now, for Golarion, it seems to be a much more "cartoon-fantasy" (I hesitate to say 'anime', because it isn't...but has elements of it, for sure). A group of guardsmen out patrolling the borderlands of Brevoy and Numeria I see men on horses. Men all of imposing stature (at LEAST 6'), with strong builds and chiseled features. They all ride massive horses, clad in scale barding. The men themselves wear banded mail, thick and imposing, with etched runes and prominant spikes and/or rivets. Swords hang at their sides, each looking like it weighs 20lb or more, and each in an odd design, with jagged-teeth along the middle, tapering out to a curved point. Daggers, likewise fanciful, on their other side. Shields are huge; easily 2' wide by 4' high, with a strange shape...perhaps a inverted curve at the bottom, and the top two corners flared out like horns from some fantastical beast. The knight is in full plate; etched in runes and phillagree, with huge spikes and a massive chest piece that is carved in the face of a smiling demon. His bastard sword is heavy...looking like it weighs at least 50lb or more, and with a few artistic holes placed through the middle of the blade and two jutting spikes near the pommel. No shield, but his gauntlets look like they could be used as make-shift bucklers from their sheer massiveness. The ranger is in studded leather, black as night, with far more buckles than any sane person needs. A dozen slender daggers are latched to the front, side and arms of this man, who can barely be seen under his flowing black cloak that seems to suck in all light around him. His bow is in the shape of a crecent moon, with the gripping area of the middle being protected by a brace, with spikes, of course.
That's the difference. Greyhawk is more "realistic fantasy, with fantasy-logic applied" and Golarion is more "fanciful fantasy, with logic ignored in favor of looking cool".
Both are fun. I like both.

Other than Greyhawk, Golarion is my current favored world...I really like the diversity and the fantastic nature of everything. In fact, I just started a campaign in Golarion, using the Dark Dungeons rules (we just don't much care for 3.x style rules)...and the campaign is specifically geared towards the PC's just wandering around checking out places and things that peak their interest. Should be a lot of fun!
^_^
Paul L. Ming