Which has less energy loss - AC step up/step down transformers or DC to DC step up/down converters?
I don't hear much of DC to DC converter modules, even though DC is
  widely used today, mostly in smaller electronics.


You might not have heard much about DC/DC converters because they are often built into the device. The vast majority of mains powered 'smaller electronics' today have switching AC/DC converters to produce low voltage DC from the AC mains. A switching AC/DC converter (also known as a switched-mode power supply or SMPS) consists of a mains voltage rectifier and filter, followed by a DC/DC converter. Many devices also have on-board DC/DC converters to further reduce voltage.   

Mains frequency transformers are used less to power 'smaller electronics' today because they are more bulky and less efficient, especially once the output has been rectified, smoothed and regulated. Although the power loss in an individual device might be so low that the consumer doesn't consider the cost, there are so many of them that the total loss could be significant.       

Transformers are used mostly to increase or decrease AC voltages for mains transmission and supply. Large transformers are more efficient than small ones, and cheaper and less complex than using a DC/DC intermediary when the the input and output are both AC. However at the other end of the scale, very long transmission lines and undersea cables have less loss using DC, even after DC/AC conversion at each end.

So in deciding which is more efficient you have to consider not just the component that changes the voltage, but what associated losses may occur in other parts of the system. In the case of 'smaller electronics' seemingly insignificant transformer losses could become a big problem when DC conversion losses and power factor are taken into account.

Efficiency Standards for External Power Supplies


  In the early 90’s, it was estimated that there were more than one
  billion external power supplies active in the United States alone. The
  efficiency of these power supplies, mainly utilizing linear
  technology, could be as low as 50% and still draw power when the
  application was turned off or not even connected to the power supply
  (referred to as “no-load” condition). Experts calculated that without
  efforts to increase efficiencies and reduce “no-load” power
  consumption, external power supplies would account for around 30% of
  total energy consumption in less than 20 years.