I'd rather do the "three times a week" routine. On all but the best shooting days, I'd probably be worn out by the time I got to 500 rounds and getting sloppy. If nothing else, my form would start to suffer.
Since I don't see a need in the future to put 500 rounds downrange in a single session, why train for it? I'd rather do 100-200 rounds, concentrating on my form, breathing, target picture, etc., and then just go home afterward.