Armory has better multisig support and it uses Heirarchical Deterministic wallets so that you only need one backup ever to backup all of the private keys in your wallet. It also supports having multiple wallets and watching only wallets.
It uses bitcoind, too, right? So, is it easy to use simultaneously with Bitcoin-Qt?
No. Since it uses bitcoind, there are locks placed by bitcoind on the data directory so you can't use the gui wallet. You can however use the command line if you really wanted to. When Armory is not running you can use Bitcoin Core as you normally would.
