If you really need an online wallet, blockchain.info is far batter than coinbase. They let you control on your private keys.
coinbase is a centralized bitcoin web wallet. We do not have 100% control on our coins in coinbase. If coinbase will become bankrupt (like mt.gox) then we'll lost all of our coins.
Coinbase began supporting 2-of-3 multisig wallets about 8 months ago where you control 2 of the 3 keys. Unlike blockchain.info, there have been no known security failures (yet) with Coinbase wallets.