XFELs are able to produce much more intense coherent pulses, with more energy and on a shorter timescale than a Synchrotron (like Diamond), this allows us to look into materials and processes on the attosecond timescale. Something that cannot be done at a Synchrotron. So in this regard they are the next generation of these machines. However because the science that can be done on XFELs is very different than on a Synchrotron, they are actually complementary machines, which would operate at the same time.