Good question. We do not recommend using the 150 pad on the ETS EC 125 sander or the 125 pad on the ETS EC 150 sander.
Although the 150 pad seems to fit on the ETS EC 125, the spindle heights on the two sanders are different. As a result the 150 pad won't engage the brake pad of the ETS EC 125. Therefore, we do not recommend doing this.
The 125 pad won't fit on the ETS EC 150 at all, because the ETS EC 150 has a keyed arbor/spindle. (See photo.)
(Tyler)