GPIB, RS-232, or USB?

I would like to purchase a programmable power supplies for our lab. Some power supplies have an RS-232 port, others have a GPIB port, While other still have a USB port and an RS-232 or GPIB port.

Is one protocol better than the others? If not, what are the advantages of each protocol? And why did NathanT recommend GPIB over RS-232 in this thread?

Thanks!