Bitcoin -qt or Bitcoind - which one to use for building a trading website? I'm building a Bitcoin website that will be fully based on wallet services and also trading. I learnt through my searching in the web that Bitcoin qt is the friendly UI version of the software while Bitcoind is just the headless daemon, but same core functionalities.

Now I was given a project to build a website that provide wallets and exchange service for users and also with trading application. Which of this two software will be the best to download? The website will be on VPS account with Godaddy.  

Thanks to Nate and Pieter for warning on the security. I understand the risk but I will still love to have answer to that question.
You should use bitcoind. You can make JSON RPC calls to it to generate an address, or to find out what the balance of an account is. Using that, you could build an application that allows access to a song if a customer pays for the song, or something like that.