SLvsKOK
What I don't tell many people is that I actually ran my campaign in the Scarred Lands before I ran it in the Kingdoms of Kalamar. And, of course, a campaign is what the DM makes of it. But if you are not looking for a campaign to buy and then fiddle with, just to use as is, the two are different enough that you may certainly prefer one or the other or prefer them both for different reasons.
The Scarred Lands' entire mythos is based around the blood of the titans. The world is dark and is steeped in the supernatural based around this blood.
Kingdoms of Kalamar focuses on the human races and their relationship to the world. There are all the standard fantasy races that you've come to love in D&D (and some extras) but humans are the favored child. The world is built around their rise and their political jockeying.
Although I enjoyed my characters in the Scarred Lands for the short time I played them (Sor 7/BW 3 IIRC), it just never fit my style of play. I prefered the peasant to lead a life of toil and tribulation because the crop didn't come in the yield he expected, rather than there was a necromancer who ravaged the countryside (this is intentionally a broad generalization to show a point).
I would like to point out a few things.
The Kingdoms of Kalamar has one core rule book (although there are several optional books carrying the Kingdoms of Kalamar label) while Scarred Lands requires you to buy several books to play in the setting (The Ghelspad atlas, both Creature Catalogues for the Scarred Lands creatures, and a whole slew of other products).
The Kingdoms of Kalamar has TWO main core books, the Campaign Sourcebook and the Players Guide. The former only has three pieces of actual 3E material (and they're all horses). The rest of it is description of the setting (I love that book). The latter has all the rules variants, editions, classes, feats, prestige classes, spells, etc etc etc. All the crunchy stuff.
Now, I ran my SL campaign with Relics and Rituals and the CC. I set it on an overlay of modernday Earth with some ecological changes. Maybe it wasn't "as much" of a scarred lands campaign as it could have been if I had owned all the books, but I don't think it's necessary to have to own them all to use the setting (although it may add to the depth of the setting).
KoK has plenty of other books to buy including its own Atlas, a book on Orcs, Villain Design and upcoming books on Hobgoblins, Cities, Monsters and a lot of other stuff. So I think the comparison in that respect is a little misleading.
So, after all that talk? I prefer the Kingdoms of Kalamar. I even prefer it over the homebrew I spent six years making. But it's because it fits how I like to game. You, on the other hand, might be totally enthralled by the world of darkness contained in the scarred lands and the wealth of opportunities titan's blood may offer your campaign. That's for you to decide.