British Steel has some fairly good songs. It's a good album, but a lot of the commercial "classics" on it pale in comparison to the more timeless songs on other Priest records. It does have some really strong tracks too, like Rapid Fire, Breaking the Law, and Steeler, but even then, nothing on it absolutely blows me away.
Number of the Beast on the other hand, while not Maiden's greatest, can at least claim Run to the Hills and Hallowed Be Thy Name, which are very powerful songs, so I'd give this one to Maiden.