I think casual users should avoid the Satoshi client. Gigabytes of blockchain data is not user-friendly, and we've done a lousy job of making it hard for users to lose their keys.
I think the something like the blockchain.info web wallet or Electrum is a good choice for long-term storage; you keep control over your private keys, and are exposed to possible theft risk only when you make a transaction (because a hacked blockchain.info could feed you evil Javascript, or a hacked Electrum download server could feed you an evil executable). The chances that you will be one of the first customers who make a transaction after they were hacked, before they took their site offline to recover from the hack, are pretty small if you are only making a couple of transactions per month.
I'm also assuming that a casual user isn't storing thousands of bitcoins. I don't think we have great solutions for casual users with thousands of bitcoins yet (I consider paper wallets a fair solution, not a great one).