The difference being, Siri "responds" to you. Google's you just input commands, and it does it. Siri is MUCH different.
For instance, my friend said "give me a *******" Siri responded, "Cannot give you a ******* at this time". It's almost like those annoying GPS devices (Tom Tom). You can even change voices. Seriously, you can carry a conversation with it.
But there is already a HUGE bug.
Even when the phone is locked, Siri can respond to commands, send email, and text messages. Without a password/passcode.