You can now download Mist as a standalone application from here.
Old answer:
The two are actually the same application currently, but Mist is under heavy development. You can run the Ethereum Wallet in Mist mode, just start the Wallet binary with --mode mist.
You can find the latest version here.
For example on Mac, run /Applications/Ethereum-Wallet.app/Contents/MacOS/Electron --mode mist in Terminal.