According to the Binding of Isaac Wikia:
Poop is a randomly generated item. The unlockable character ??? starts with an item that can generate poop. It randomly drops coins or hearts.
That said, I've made my way through the depths of Isaac's world more times than I care to count, and speaking from experience, have only ever seen Hearts (Regular and Soul), and Coins (pennies and nickels) drop from these piles of poo.
As for poop productivity, while I don't know the percentages, it doesn't seem like Larry Jr.'s poop has a higher chance of dropping anything. However it is a seemingly endless supply of poop. As long as you can stay alive, you can play in the poop all you want in hopes of farming some money or hearts.
One important thing to note, however, is the Lucky Foot. Unlocked by collecting 55 coins in one game, it will begin to appear randomly within the dungeon (treasure rooms, boss rooms, and Devil rooms). This little gem increases the likelihood of items dropping from those putrid little piles of poo.