I have the same sentiment as leakyroof above, in my world there's always room for another Festool sander. Well, to some point, but way beyond a single one. I have 8 of them right now after deciding to downsize a bit and get rid of the lesser used ones.
I agree with the posts above that the RO150 is a nicer sander than the RO125. If you go Rotex anyway, go for as much power as you can get because that's where it shines. And according to lots of people the 150 is more stable and easier to handle than the 125. Anyhow, you'll always be able to work faster with the 150 than the 125.
I also have the ETS150/3 and ETS125 and I use the 125 a lot more than the 150/3. I like how it is so small and prefer to use it on anything the size of a door and smaller. It is my door specialist. Only on bigger surfaces I see a time gain for the 150/3 but that doesn't happen a lot and I also have the Ro150 for that so I think I'm gonna let go of the ETS150/3 soon.
People often mention sharing papers between sanders, and I guess that matters when you have to buy 100 packs. I almost always buy 10 packs so sharing papers is not important for me. With 8 sanders you're still going to end up with a lot of diversity.