- Festool emailed that the form factor is "about the same" and the main difference is pad size (though not the only difference?)
Language nuance, per Cheese. You can look in the eKat spare parts catalog the parts lists ..
- Local dealer said the 150 grip is slightly larger but also said the 150/3 was more aggressive than the 150/5
Both false.
- Some people online have said the 125 and 150 are weighted/balanced differently to accommodate pad size
Correct on the letter, though the difference is negligible in practice. Many using the 150 pads on the 125/3 sanders daily for years. If you ever held the pads in hand you will understand why. They are very light on the outside perimeter. The difference when using a protection pad is probably more than between the 125 and 150 pads.
You can check in eKat, the "fan assembly" is the part doing the eccentricity as well as the balance.
- Other online comments mention that Festool said using the 150 pad on the 125 may wear out or damage the break indicating break differences
A misunderstanding, probably assumed from other vendor sanders design. The ETS EC series does not have a mechanical brake but an electronic/powered one and the motor assembly is the same.
Advice:
If you are a hobby user, stick to the /3 sanders and consider pairing it with the ETS 125 with the edge guide down the line, so I would go with the 125/3 and immediately get the 150 hard and soft pads for it and some abrasives.
The main advantage of the 150 is the bigger coverage area hence faster standing of flat surfaces. If you do not have another sander for that, you want to use the 150 paper as the default.
Also, make sure you have a good dust extractor to go along. These are Pro sanders and cannot work without dust extraction as the dust impeller is skipped /to make the sander lighter/. If on a tight budget, the CT 15 is a very good cost/value proposition. Otherwise the consensus here is the CT MIDI is the "best first buy" for a hobby user with an unequipped workshop.