His high mid register had more power around 1984-1985, and he applied a better vibrato to my ears back then. Superior, actually. On the other hand, it sometime sounded pretty strained live, while he nowadays seem to stay clear of that by singing those notes in a different way. Moreso, Bruce's grown more consistent live since in general, and the mellow side of his voice is much better now.
So. For me it comes down to Powerslave or Brave New World. I'm not quite sure how to choose between them, as we're talking different styles.