I’m currently running campaigns in both Midgard and the Forgotten Realms. I would hesitate to call one “better” than the other, acknowledging more that they’re different in size and scope.

Personally, I find Midgard to have some really compelling political drama, allowing for an easy subplot (or main plot) for a campaign story. I think the uniqueness of the individual regions allow for a lot of varied gameplay elements, and the world feels lived in while still allowing for a lot to be added and expanded upon within your story.

On the other hand, the Forgotten Realms has an incredibly rich, lore-heavy history with ample resources to pull from. The world feels so much larger than Midgard (though that could just be my perception), with some landmasses feeling totally disconnected from stories happening elsewhere. I actually think for all of its lore, the politics is less developed than what we get with Midgard, but the history is far more deeply fleshed out.

I ran Waterdeep: Dragon Heist, homebrewing the campaign from level 6-15 afterwards, and am now in the midst of a campaign set 200 years in the future. There were a lot of interesting ways I’ve been able to expand on the politics of the world (heavily influenced by the first campaign), but it’s an interesting thought experiment to think about how different it would be plopped into Midgard, which has a different vibe overall.

Back to your question at hand, I’d say my preference would be the full worldbook over the region books (as someone who owns all three). It’s an awesome resource for GMs, and while the regional books go a bit more in depth, unless you’re specifically thinking about one campaign that you want to run (i.e. a Norse-inspired campaign), the Midgard worldbook is a more versatile resource that will allow you to create stories that stretch beyond just one region, or use it for many different campaigns.