When you partition a disk it doesn't divvy the drive up by platter, it divvies it up by cylinders (tracks). This is to minimize head movement.
For two drives with the same recording density (bytes per platter), having more platters gives you more capacity (obviously) and can also improve performance since you can access twice as much data without having to move the access arm.
You could take the 1TB disk and partition it so that it only has a single 500GB partition. If you did this, the 500GB partition would be distributed on both platters and located on the outermost tracks of the drive (probably about 1/3 of the tracks, since the outer tracks have greater capacity than the inner ones). This would mean less head movement and higher average transfer rates, both of which would contribute to better performance.