Predictive dialers are much better and advanced compared to the simpler progressive dialers. This article differentiates between the two of them.
Predictive Dialers are auto-dialers that make automted calls to numbers from a list, and once the calls are picked up, the dialer connects them to a free agent, who can then talk to the person dialed. In addition to this, predictive dialers have the ability to predict or forecast when the next call is likely to be connected (excluding all the phone ringing, busy tones, and answering machines encountered), as also it can estimate when the next agent is likely to finish a call and be ready for the next call.
Because of this prediction capability of a predictive dialer it can save time, by starting to dial new numbers from the list, while all the agents are still occupied on their previous calls. This helps in saving a lot of time, and increases the call centers' productivity.
Progressive dialers on the other hand, are not capable of making any predictions. What progressive dialers simply do, is that they auto-dial numbers one after the other, so that an agent does not have to manually dial the numbers. The dialer waits till an agent becomes free, and then progresses onwards to dial the next number on behalf of the agent.
So, while progressive dialers do help in automating the process of dialing, they cannot save the dead time that is spent in-between calls in the way a predictive dialer does.
Predictive dialer softwares are better since they allow for more number of calls to be completed in the same amount of time, but they are also costlier compared to progressive dialers.