I should clarify that I don't know for a fact that the sensors operate differently, just that they appear to be different (the Sport model sensors don't have that [liquid?] metal circle around them) and the sensor pack material *is* different, so logically, there should be *some* difference in function. Zirconia ceramic is usually chosen specifically for it's radio-transparent qualities, but Apple may have used it here just because of it's durability or whatever.
The only review I've read that mentions anything about this, said that the Sport models with the rubber bands fit more snugly on the arm, and therefore the reviewer thought that the heartbeat readings were actually *more* accurate than the steel watch. Who knows though? I'm sure the difference would be minimal at best, but they are different.