Both settings are very good but take very different approaches. There really isn't a bad choice to be made here.
Kalamar is similar to Greyhawk back in the day - info on gods, nations, cultures, etc. It's one of the most internally consistent RPG settings I've ever found, with probably only Harn surpassing it. The campaign setting was littered with plot hooks but it wasn't a book that lent itself to sitting down and reading it cover-to-cover. Most criticisms of the setting I've seen are that it's "vanilla" or "dry" but in my experience that's the assessment you can get from skimming rather than reading. If you read through it, there's plenty there. In fact, more of the fantastical is highlighted in the core setting than Greyhawk had back in the day.
Scarred Lands is almost the polar opposite. It's thematically rich and epic in scope - gods vs. titans, global upheaval, a mix of standard fantasy & Greek age of legend. It's impossible to read through just about ANY Scarred Lands book and not have ideas spring to mind, and your toughest choice is probably which one(s) to focus on. HOWEVER..., internal consistency is NOT the setting's strong suit. Climate, cultural, national, and economic interactions reveal glaring, gaping holes on even the smallest inspection beyond a cursory glance. Things exist "for story purposes" with little or no consideration of larger implications on the setting. It's a setting that would seem to be points-of-light given recent history but most of the machinations/interactions are done on a continental or global scale/view.
Scarred Lands is like the Marvel Cinematic Universe with less attention paid to setting consistency or connective tissue between movies - it's epic and great for a summer blockbuster. If world-shaking events and high-stakes action are more important than world-building and campaign longevity, it's great. Kalamar is more like Netflix's Marvel TV universe. The foundation is there for more grounded, character-driven stories. It can do big-scale set pieces and sweeping events but lends itself to building them over time and you, as the GM bring them to the sandbox rather than them being placed on a tee for you.
I discovered both of these settings around the same time and agonized for months which was the better option. I initially attempted to run with the Scarred Lands but for the more sandbox-oriented campaigns that I run, the lack of internal consistency ultimately was a deal-breaker for me. I ran a successful Kalamar campaign for several years. It involved dwarven resistance fighters waging guerilla warfare against Kalamar's Roman-empire-in-decline equivalent and the setting provided me with a strong foundation to frame the campaign while still providing massive room to incorporate my own stuff.
I've since moved on to other settings. First Golarion, then Midgard, and finally arriving at the Lost Lands. When I'm feeling uninspired or have writer's block, I still break out my Scarred Lands books from time to time to get the creative juices flowing again.
