What is the difference between the two sensors?
Apart from the obvious construction and component differences the two sensors seem totally equivalent from their look.
They seems to both be FC-51 modules: those are “line followers” meaning one diode (the clear one) emits infrared light and the other (the dark one) measure the amount of IR light reflected by the surface in front of them.
The potentiometer is there to determine the triggering level so that when powered you can have a LOW signal on the OUT pin when the threshold is reached.