With the three cores initially spread out over three months, KotS looked like a much better deal in terms of a preview.
I'm still buying it. Why? This is going to be the first look at what a 4E adventure is supposed to look like. I admit I haven't looked at any of the fan created modules, but I'm sure they did the best they could with what they had. They did not, however, have access to the full rules. They don't know exactly how skill challenges are going to work, about noncombat stuff, about traps and what goes in to making an encounter location exciting and dynamic, and how all of this fits together. Yes, two weeks after I get KotS, I'll be able to learn all of that, but KotS is going to serve as a blue print as I start making my own dungeons and encounters.
I'm also looking forward to KotS for the preview aspect. Based on what we've seen, I expect some of the classes and races are going to feel very different. One of my players likes rogues; she might find the 4E rogue really doesn't fit, but that the ranger is more her style. If I can use the two weeks to let the group mess around with the KotS pregens, maybe they can get a better idea of what they want to play, and I can avoid any do-overs for character creation.