I used to order from puritan now i do that from iherb..you get a 5% off if you order over 60$ and also 5$ off for first time customers..Also I prefare the brands there (nature's way,natrol..)They have product reviews too which will help you chose..Personally i liked iherb way better than puritan.
swansons and puritans ran a tight race, both had good shipping. I only compared a few products that I ordered.
Personal preference would be what to watch for in mg. I cannot speak for the quality. Quantity of pills per price were very close. Definitely worth the time of comparing both sites before ordering.