Scarred Lands is good if you want a setting that sets everything up for you, fills in all the blanks, and gives you a complete package with a strong thematic hook to base your campaign on (standard, but well done, apocalypse hook). This hook can be overbearing if you want to focus on your own ideas.
Kalamar is good if you want a world that focuses on providing great detail (have you seen the Atlas?) but remains flexible enough to handle any kind of campaign you want, any kind of flavor, and any type of adventure. Instead of one big event defining the entire world (ala Scarred Lands), lots of very interesting and very different things are happening all over the world. You get lots more flexibility this way, but because it lacks a strong central theme/hook some people find it boring and harder to grasp immediately.
In all, Scarred Lands focuses more on giving you completed pre-packaged plotlines whereas Kalamar focuses more on giving you open ended tools to invent your own plotlines. I think what you choose should depend on your DM style.
Scarred Lands is a bit higher magic/fantasy than Kalamar. Kalamar is very well suited toward political campaigns, whereas Scarred Lands is very well suited toward save-the-world-from-evil campaigns. Kalamar expresses more of a moral relativism than Scarred Lands. In Kalamar it is often not clear at all who is good and evil -- in fact these terms are wholly irrelevant in most of the world's conflicts. In Scarred Lands, you are pretty much told who the good guys and the bad guys are.