Both brands make a double-frame toploader: two rigid plastic sheets welded on three sides. Both are the "default" answer depending on which shop you walk into. The differences that actually matter are smaller than the brand loyalty around them suggests.
Where Ultra Pro wins
Recognition and availability. It's the toploader most local shops stock first, which matters if you're buying in person rather than online. Quality is consistent pack to pack, and it's been the standard long enough that there's no guessing whether a size will fit a specific card.
Where BCW wins
Price at volume and consistency are the practical differences. BCW often comes out slightly cheaper when bought by the case, while local availability can favor Ultra PRO. We use a penny sleeve first with either brand so the card surface does not rub against the rigid holder.
What neither does
Neither brand's standard line includes UV protection, and both show the same visible weld seam from the double-frame construction. If either of those matters for a specific card, that's a reason to look at a premium brand like Max Pro, not a reason to pick between these two.
Shop BCW on Amazon → (paid link) Shop Ultra Pro on Amazon → (paid link)