I have a BH-505 but have never used a BH-503. The fact that the BH-505 is an in-ear headset helps it block out external noise and increases the bass response.
The BH-503 looks a bit too loose fitting like old school earphones. It's not easy getting the perfect position for the BH-505 but once you do then the output is amazing!
heavy movements like walking down stairs, running or fat walking will cause it to come loose; they will probably never fall off but they move out of the ear slightly then you loose the bass and external noise can then be heard.
I can only keep mine on for a little over 2 hours at a time, they get uncomfortable after a while.
Show your appreciation. Hit that kudos button real hard