We don't have either product to compare, really. Radiant Citadel is 224 pages long, and encompasses 16 different microsettings, with 13 having specific Adventures. Spelljammer on the one hand more encompassing, but per Perkins that will not be going in depth into Settings other than Wildspace and the Astral Sea in general. So Spelljammer, as a Setting to itself, is getting more detail than any part of Radiant Citadel, most likely. But we'll see what we get soon.
It would be good to remember that these products are doing different things.
Radiant Citadel introduces us to an entire setting and presents a series of adventures n that setting, and the highlight of that setting is how different everything is in each segment of it. They're covering massive ground, and it is mostly entirely new ground.
Spelljammer, however, is updating an existing and well supported concept. Do I need a battlemap of a Nautiloid in the product? No. because I can use Google and find dozens out there. Do I need lengthy ecologies for Neogi, Giff and Thri-kreen? Only if they want to make them differ from the lore established in the 1990s. Otherwise they give us the minimal version and we Goggle the rest. Unless they're changing the lore, the majority of the Spelljammer lore can be presented briefly in the books to facilitate Google searches (or just flat out DM's Guild purchases of old material - I could 20+ products there).
In terms of locations detailed, they will probably be about the same, one with a chapter detailing the Radiant Citadel, and the other will have a chapter detailing the Rock of Bral.
But Spelljammer is a book about traveling between worlds, not describing the contents of the entire multiverse. In the 5e sense "setting" means "type of story" not rattling off a long list of locations.
Do not expect the locations in the Radiant Citadel adventures to be detailed. Candlekeep Mysteries goes to places like Tashluta and Anauroch, but it only has very cursory information about those locations (and no map).