I think both are important, although a lot of the mundane detail I’d rather have in detail, which requires a full Tribunal book or a mundane book on the topic (I recommend Bennett and Hollister’s Medieval Europe: A Short History).
I see a continent wide book fulfilling four purposes. One, a general introduction on each tribunal; in-game this would be information any magi outside the tribunal would have easy access too, out of the game this serves as general information for players and something of a teaser. Two, the book would cover some of the inter-tribunal and Order-wide issues that don’t entirely fit into the Tribunal books. Three, it allows the variety of the Order to be showcased (more important for newer players, but useful for everyone). Four, it allows tribunals covered by previous editions to get a small update. I figure you could probably have an introduction, 9-12 pages for each tribunal, and then probably a chapter or two about the Order as a whole and saga ideas. That should result in a book between 150-200 pages long.
With eight pages I see something like the following:
1 page introducing the region, discussing the general environments and important sites.
1 page going over landmark events of the tribunal’s history (mundane and mythic)
1 page going over the current mundane political and social landscape
2 pages on the hermetic landscape, particularly notable hermetic characteristics
1 page overview of the major covenants
1 page looking at a specific covenant in detail
1 page looking at a specific magi
1 page of saga ideas
Across thirteen tribunals a variety of covenants (all four seasons, nomadic, monastic, urban, faerie, highly magical, politically powerful, politically weak, et cetera) could be featured as well as a magi from each house.
Getting a book like this to work well with the established books is admittedly tricky. You want to minimize content that is effectively an excerpt but at the same time it wouldn’t make sense if its all new. Chapters covering a tribunal that has a 5th edition books can probably be minimized for this reason, although they’re also an opportunity to feature less important or weaker covenants and magi.
For example, let’s take the Normandy Tribunal.
You’d want a bit about the Carolingians, Capetians, Philip II, Louis VIII, and Paris. Hermetic history would likely focus on the schism war and the squabbles that followed. It would probably be worth doing a paragraph overview of each liege lord. Hermetic issues of note are the scarcity of vis, contact with mundane, covenant vassalage, the tourney, and a little bit about raiding. A covenant from this tribunal is tricky, since you don’t want to be redundant with an existing 5th edition book. Atramentum Renatus is an option. As a vassal of a vassal it features that system nicely, and it isn’t discussed in much detail in the book. Spider’s Palace might be good to tie it into a later discussion of the Lotharingian movement (although a Fengheld chapterhouse might be better for this). Another option is a small covenant not covered in the existing book.
Exemplum, last in the line of Sinapsis, and formally (?) vassal of Lapis Crudus. It was a young spring covenant when its liege disappeared, and while this opened up a number of resources it lacked any political might or influence. Exemplum was a frequent target of raiding, particularly from Atsingani. It floundered for decades, never actually falling apart (thanks to its vis sources) but never really establishing itself either. Around the time it finally started to settle and fully enter summer it’s liege re-appeared, leading to considerable debate over what to do about this. Exemplum is an example of a politically weak but marginally wealthy covenant, it touches upon several distinct elements of the tribunal, and it has hooks to covenants detailed in The Lion and the Lily. Top it off with someone with Diedne blood to be the featured magi of the chapter.
As an example of an earlier edition tribunal, let’s consider Stonehenge. The hermetic section likely focus more on politics (Blackthorn versus Voluntas). Goliard of Blackthorn would be an interesting maga to feature, she doesn’t appear with statistics in Heirs to Merlin, and she likely gives you hooks to the Hibernian tribunal (through the Ashenrise covenant) and Transylvanian tribunal (through her connections). As for a covenant to feature, Libellus and Nigrasaxa are both blank slates and either would be a good example of a spring covenant.
Done right, and I realize this is a much easier thing to say, I think the book could be a convenient resource for established players, give new players a good general grasp of mythic Europe and the OoH as a whole, and provide hooks to other works (and not just the tribunal books).
A book focused on just one city would be neat, but I think something featuring three or four might be a better idea. I think coming up with one single city that works well for lots of players would be difficult to accomplish. Constantinople would be neat, although it's already featured quite a bit in the Sundered Eagle book, although I suppose any major city is likely to be featured quite a bit in their respective tribunal book. (And a minor city would be an odd choice, although still possible.)
Several cities removes some of the intricate detail, but I think it makes for a more flexible overall work. It also means I wouldn't have to go over my distaste for the romanticized cloaca so many of you are discussing. 
(I'm not sure if it would have smelled better or worse in 1220 than it did when I visited. Although I do imagine the food was better back before it became a tourist trap. . . . although I imagine it already was a tourist trap by 1220.)