What are the main differences between these two different setting when I set up a connection in your portal?
Those settings define the way the switch delivers calls to devices represented by the IPs, in a similar way a PBX delivers calls from a queue to an agent.
So, sequential means that when a call has to be delivered, it will be first sent to the first user agent/IP in the list, than if that destination is busy or unavailable, it will be sent to the next one and so on. This is the best option to configure a failover destination, as the calls will always go to the first IP, unless it is unavailable.
On the other side, round-robin, will distribute calls through the UAs/IPs of the list by taking turns. It will start from the first one, and will remember that position, in order to deliver the next incoming call to the UA/IP which is next. This may be used as a load-balancing solution, because calls will be distributed almost evenly through all the available destinations.