Is Nokia bh 503 is better than Nokia bh 504?

No. The bh-504 is better than the bh-503. The differences are in the isolation and volume which are both much better on the bh-504. For more details visit this link, the reviewer had both bh-503 and 504. Hence being able to give a full comparision of the two,

howardforums.com/showthread.php/1573004-Nokia-Bluetooth-Stereo-Headset-BH-504-review