The
ETS 125 is a dedicated random orbit sander. It is best used for more detailed finish work. It is not nearly as aggressive as the RO.
The
RO 125 is a dual mode sander that handles Rotex (rotary motion) and random orbit motions.
A little more detail on the types of motions. A rotex or rotary motion maintains the entire pad spinning even under load, creating a very aggressive action. A random orbit motion is similar, but not quite the same. The pad is free to spin in a rotational motion, but when put under load it will usually stop rotating and only continue orbiting. This results in a less aggressive action
Some over simplified images to help you visualize:
Rotation
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Random Orbit
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The ETS being less aggressive should be easier for you to control, and get used to. But be sure to note it is less aggressive and will not be as effective for rapid material removal.
Ken