The main difference is how a call is connected to an agent. For auto dialer, the recipient must press a key in order to connect to an agent; while for predictive dialing, the call connect is automatic for live answered call. This difference makes the usage model quite different from the two dialers.
Predictive dialer requires agents sit in front of their computers. If a call is answered by a live human, the system will show a pop up window on an agent's computer. The agent should answer the call promptly, otherwise the recipient is likely to hangup the phone.
Auto dialer, on the other hand, can expect the recipient wait for the phone to be connected to an agent. So the transfer does not have to be immediate and thus possible to transfer calls to a cell phone. This allows agents not to sit in front of computer in order to answer the call.
Normally, auto dialer plays a pre-recorded audio message after a recipient says hello. However, Voicent predictive dialer can do the same. The only difference is that in predictive dialing mode, the call connection is automatic after the audio message is played.