By any comparison, Sigil is better positioned as a trade city. It has portals leading anywhere, plus many permanent portals well known and exploited by various planar organizations, trade groups, etc. It doesn't require transit through an environment inherently hostile to a majority of lifeforms like the Elemental Plane of Fire (or Elemental Chaos). Everyone and their brother wants to conquer Sigil, and it's only the fact than any attempt leads to mazing and/or blood-spattered death by Her Serenity that keeps it from being a constant war zone. The CoB hasn't ever really had any major attempts to seize it, probably because of its location and its relative comparison as a trade city to any number of cities on the planes like the City of Glass, Shrak'at'lor, Dis, Tradegate and any of the other gatetowns, etc. It's good, but it's nowhere near the best.
The CoB is rather hot compared to Sigil. Sigil may have portions heavily cloaked in fog, mildly acidic rain, and for the Lower Ward a ton of soot and ash, but it's a hell of a lot more receptive for most merchants than a city surrounded on all sides by flame, where water is even more scarce than Sigil, and life is generally unpleasant for non-natives, etc.
Plus, the Efreet have a rather more unreceptive sense of laws for the city than Sigil (except for the Hive). The CoB's legal system is probably comperable to Sigil's if Sigil's was entirely run by the Mercykillers.
