The 500 has a built-in, metal cam stack for zz and some other 20+ decorative stitches controllable by a pair of stacked knobs on the face of the machine . It also came standard with 5(?) plastic cams for additional decorative stitches. The 503 is straight stitch only without the 20+(?) plastic cams for zz and decorative stitches that can be inserted individually into the machine. Both have the same motor, Singer slant needle, and 66 bobbin configurations; and are well regarded. Their stretch/knit fabric stitch selection is a little weak (primariliy zz) compared to more current machines.
Around my neck of the woods, I seem to run into more 500's than 503's which run from $15 or so to $250 (way too high for me!), depending on condition, cabinet, accessories and price goal of the seller (realistic or not).
John Thomas in NC