If either are used in the body of a Solidity function that may change the state of a contract (write call), what's the primary difference and/or benefit of using one over the other. For example:
function setOwner() {
owner = msg.sender;
}
vs.
function setOwner() {
owner = tx.origin;
}
What's the difference?
With msg.sender the owner can be a contract.
With tx.origin the owner can never be a contract.
In a simple call chain A->B->C->D, inside D msg.sender will be C, and tx.origin will be A.
msg.sender is preferred for the flexibility it provides. Furthermore, for Serenity, even though it's a while out, Vitalik recommends avoiding tx.origin: How do I make my DAPP "Serenity-Proof?"
Carefully consider if you really ever need to use tx.origin. Remember, you may not be the only user of your contract. Other people may want to use your contract and want to interact with it via a contract they've been written.
If the origin is really desired in D, then each of the functions in the contracts B, C, D could take an extra parameter to propagate the origin: A would pass its address (this) to B, B would pass the value to C, and C would pass it to D.
EDIT: To emphasize the comment by @WBT below, a contract that uses a passed in value for the origin, must be very careful in how it uses the origin: anyone can pass in a value that is not the real origin.
msg.sender gives the direct sender of the message, so for example a contract that passed it along.
tx.origin gives the origin of the transactions, so the user address it was originally sent from. In practice this will always be a user so eth's answer holds true.
