I have the rotex 125 (my first festool sander), and the ETS-EC 125.
Far as size, 125mm sand paper can be found in any store in a pinch, 150 is no where to be found. I also just had no reason for a really large sander. It's not like I'm trying to polish a sailboat every weekend.
I first bought the rotex thinking it could be the do all. As others have mentioned, it can be near impossible to control, with no pattern or reason to why it suddenly goes crazy and tries to rip your arm off. It can be going along just perfect and suddenly just take off when you are going everything the same as you were. Some times it perfect all day long, the next day doing the exact same task it is completely uncontrolable. Some people try to say it's how you hold it, I've held it in every possibly way, same results. I personally think there is something in the design (gear train probably), that gets into some funky mode, possibly a resonance with the material being worked. I would say the tool does get into the dangerous category, I'm not a small person, and it has seriously wrenched my arms/shoulder body when it decides to take a random exit path from where you are going. Still, it definitely has a place.
Anyways, because of that I bought the ETS-EC pretty soon after, and as many folks will tell you, it's a wonderful sander.
What I found is there is a place for both. I use the rotex when you need to remove material, shape a wall stud, cut down a tree, smooth down a mountain range, etc. For general sanding, I use the ETS-EC, but then when I do finish, the rotex is great for taking down poly and buffing stuff out. I do stuff with many coats of water based poly, and work thru the process using both sanders.