A quick glance at the maps for the two worlds is enough to tell me which one to choose.
The FR map makes no sense. Mountains, forests, deserts, rivers, and swamps are in places they could not be without the use of constant year round high magic to keep them there.
Kalamar's mountains, forests, deserts, rivers, and swamps are all where they should be when you consider how plate tectonics works and how humidity and wind patterns form around mountain chains and continents.
The sad thing is that it only takes the most basic layman's understanding of these fields to get this. The kind you can learn in reading a single paragragh or two on geology.
Mountains form when volcanos go off or plates collide. Wind rushes along magnetic patterns shaped by heat from position to the sun - so it kinds matches with the seasons and has an east or west consistant drift. Humidity builds where moisture gets trapped by elevation after being carried by the winds - often giving dry regions on one side and wet on the other.
Now shift to cultures, and you see the same problems.
The cultures in FR are thrown down in a random mish-mash and make no sense when coupled with resources, logical trade patterns, and societal drift from emigration over time and the results of things like tribal merging and warfare. There is no pattern to any of it.
Kalamar starts with a group of four distinct super-cultures (no that's not a typo, it's four in the early history), and shows how they have migrated across the globe. Where they have come into contact you get cultures that are shaped by how this contact turned out and what resources were available in the areas they met. There is also consideration for how they were shaped by the non humans they met there, and how those non humans where shaped by them.
As a result every culture shows what seems to be a natural consistant evolution and seems to fit into it's region and alongside it's neighbors.
When we move on to NPCs we see that FR is littered with high level NPCs that can answer any plot you throw at your PCs, so you have to answer why they don't.
In Kalamar nobody's epic level, and few are even mid to high level. There are no iconic NPCs, becoming the spot light is reserved for your campaign. In line with this, there is no meta plot. The Kalamar timeline is frozen in place, so individual DMs can unfreeze it and move it forward as they wish. No novel will rewrite the world on you in some future update that you need to account for in order to buy future products.
Both worlds are littered with adventure ideas. In Kalamar they fill out almost every page of text and tied into the world history, cultural tensions, legends, and so on. Conflicts that shape these ideas are based on elements of 'human nature' - political, social, sexual, ethnic, resource, and economic tensions.
In FR they are tied in places, and kewl in others. Often they are wrapped in cosmic forces that make their resolution something impossible for all but high level characters. Some of those in the core books have already been solved by novels or later suppliments - in effect taken away from DMs who wish to keep 'product current'. The conflicts that shape the ideas here are usually driven by alignment, metagame concerns, conflicts between iconic NPCs and or deities, and novels written in the setting.
On magic, Kalamar is scalable - it makes no statements on the issue but assumes the standard ruleset and power levels. A DM can scale it to be high or low magic.
In FR there is an assumption of frequent and powerful magics nearly everywhere. It is tightly scripted into the setting and removing it will require a lot of work for a DM who wishes to continue using further FR resources beyond the initial book. Even the use of the initial book will require a lot of work if you desire any magic setting other than the frequent and power one assumed within it.
Where Kalamar loses out to FR is in presentation. The FR books look better. They have dynamic color graphic design with top notch illustrators. The writing is better paced for short burst reading - making it much easier to digest. And the design team and publisher have a lot more brand recognition to them.
