Take the B. A "B" is a plenty solid grade. Ws exist for reasons, but not for the ones you're citing (changing majors, don't need this class, and don't find a "B" acceptable). A "W" should be taken when NOT taking a "W" will result in a failing grade (D or F), since a D or F would probably hurt your academic record more than the W would. A "W" taken when the grade would otherwise be a perfectly respectable "B," on the other hand, will hurt your academic record more. A "W" is not to be taken lightly; it's to be used sparingly when absolutely necessary.