When it comes to small power devices, the efficiency of the converter is what will makes the difference. If the DC device was 12v, then no conversion is required and no power is lost.
If your device is say 3.5 volts, then the 12 volts needs to be dropped to 3.5 and how that is done determines how efficient it is. If it's done passively, then nearly 3 times the power is lost in the conversion. Not very good. Modern direct DC chargers (cigarette lighter socket type) use switching power supplies, not passive dropping components, and are nearly 100% efficient. (high 90s)
Some inverters are also very inefficient at converting 12 to 120VAC at low currents and then you need an AC adapter to go back to DC.
If you have modern switching power supplies/converters, then it's not worth worrying about for the amount of power used. Once the battery is charged, a direct DC adapter will be using nearly zero energy. If you use an inverter, the inverter still hums away and the AC adapter is consuming parasitic power.
30 years engineering