Yo Pojo

Well, there are many situations in which one is better than the other. Bill is pretty simple, and adds 2 cards to your hand. This is usually useful, and can help you picku up that Oak, Double Colourless, or pokemon that you needed. It is more useful than Mary when you have, say, a Computer Search in your hand, then pick up a Bill, so you can draw the cards you need to use that Search. It is useful in most decks, as it offers another method of card drawing, when used alongside Oak and Computer Search. However, it is most useful at the beginning of the game, and is not paricularly helpful at the end of a game when you are indanger of decking.

Mary on the other hand, provides an alternative method of card drawing. Mary is useful in many situations. Say you happen to be given 2 itemfinders in your starting hand (and let's face it, Item Finders seem to be ATTRACTED to your starting hand), Mary offers you a chance to draw 2 cards, and then return cards that uyou don't need right now to your deck for use later on. This is useful for getting rid of excess oaks, evolution cards, and it solves the problem when you have a Chaos Gym/Lass and Oak in your hand at once. Mary is also more useful at the end of a game because it allows you to cycle the cards in your hand, and take perhaps a better two from your deck than what you have, while not actually lowering the number of cards in your deck.

So which one is better? Well, it is probably a matter of opinion. People have said that seeing as Maintenance was never played and was deemed useless, then Mary is the same, but I disagree. First of all, with Maintenance you had to shuffle the cards in FIRST and then draw ONE card. With Mary it lets you draw TWO first, then you can choose what you shuffle in, making it much more useful, and quite different to Maintenance. I think that in Haymaker, Trapper and Wiggly decks, Bill is better because those decks require a greater amount if card drawing, and they often won't have evolution cards that they need to save for later.
However, in evolution decks such as Dark Vileplume, Stall and Raindance, Mary provides an alternative and more useful method of card drawing.

And for all you people who say that Mary should not be a rare, just take a look at Devolution Spray, Charity, and an awful lot of other trainers that are rare. Mary is no worse than any of those......