"Serial" cables were designed to work with the telco network, which wasn't Ethernet.
As to which is faster, depends on what particular variant of Ethernet and Serial you're comparing.
BTW, Ethernet is serial too, just different from telco serial. Serial comes from transmitting information "serially" down a single wire. "High" speed connections often transmitted information on multiple wires in "parallel". Generally, x number of wires allows x faster transmission, all else being equal. However, x number of wires also increases the cost of the wire by about x, which can become quite significant for WAN links that might span thousands of miles.
