The premium agents are the market leaders. They lead, introduce new features and services first, and the other's copy. That alone speaks for them.
The big difference is the service. By that I don't mean that the customer service is available for longer hours or nicer to talk to, I mean that they will reliably do what you ask them to do: Order your item in time, take the QC pics as you instruct them and don't waste time.
It's also incredible how they inspect your item and show you the smallest imperfections and ask you if you want to return or exchange it.
The main difference is that they don't mess up. That's because they have high levels of automation, technical help and robotics. Budget agents are known to mess up a shipment every now and then and send it to the wrong address. If that happens it's a total loss for you: you just lost 100% trying to save 1%-2%.
added: 6/26/2023