ThermoJet vs ThermoCoil vs Dual Boiler
ThermoCoil is a heating element wrapped around a coil that heats water as it passes through; it is what the Express and Impress use. ThermoJet is a newer, faster system that gets to brewing temperature in about 3 seconds, as on the Bambino, Pro and Touch. A dual boiler has separate boilers for brewing and steaming, so you can do both at once. PID, short for proportional-integral-derivative, is simply a controller that holds water temperature steady.
Built-In Grinder or Separate Grinder
A built-in grinder is the simplest path, and the Express family and Touch machines do it well. The catch is that if the grinder fails, you replace or repair the whole machine. A separate grinder costs more up front but keeps your espresso setup flexible.
Manual vs Automatic Milk Texturing
A manual wand rewards practice and gives you better microfoam in the end. An automatic wand, as on the Bambino Plus, Touch and Oracle Touch, gives repeatable milk with no skill needed. Choose by temperament.