This may be hard to answer, but I just want to know in which a Pokemon with surf will be used more often/ be more important.
As far as urgentness both are pretty much the same.
You need Surf to beat both games.
As far as usefulness B2/W2 win.
Unova has much more bodies of water to use Surf in. There are also probably many items that you have to use Surf to get to. You can also use Surf to catch wild Pokemon.
Hope I helped!