NameID is about storing your identity into Namecoin, BitID is about authenticating to a service by proving you control a Bitcoin address.
BitID and NameID complement themselves.
For instance :
1. you sign in on a service with your BTC address using BitID
2. the service queries NameID and retrieves the identity (name, email, avatar...) attached to this address
Sorry, but NameID does both, you're repeating work. You can log in using NameID to prove you own the Namecoin address linked to your Namecoin ID information. That's how it has worked for a while.