Solidity Contract vs OOP Class – Key Differences Explained

The Morden testnet is the public Frontier testnet, meaning there are other miners, and other people will be able to see your contracts. It is important to note that you should not use the same accounts on the testnet as you do on the live network, or you will be at risk of replay attacks.

The --dev flag creates a private testnet, accessible only to you. It automatically sets flags like --maxpeers 0 and a temporary datadir. Since only you are mining, you are guaranteed to find ether, whereas on the Morden testnet you may need to wait a bit for test Ether. --dev also increases the verbosity, turns on VM debug logging, and starts Whisper (--shh).

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.