I got my first Festool sander a few months ago. I first of all looked at the RO125, and borrowed a demo one for a couple of days. Based on how well it took material off, I realised that the rotex was a bit of an overkill for fine finishing, and since I didn't anticipate doing a lot of rough sanding I couldn't justify the extra cost. So I then looked at the ETS 125 & 150. I didn't borrow these, but tried them both out on real timber in the shop demo area.
My impressions were that the ETS125 is far from perfect. It seemed very unstable in use, and prone to wobbling/tipping. Despite being bigger & heavier, the ETS150 was far easier & more comfortable to use. When it came to the choice of orbits on the ETS150, I asked opinions here & decided to go with the 3mm stroke, for 2 reasons. 1; I wanted it for fine finishing work, and the 3mm stroke would be better for that, and 2; if I ever needed to do a lot of heavy sanding, or a corian/solid surface job, I'd end up getting the RO150 as well, which on random orbit mode has a 5mm stroke - that would then make the ETS150/5 obsolete, as I'd then have 2 150mm dia, 5mm stroke sanders.
I (so far) haven't regretted my choice of the ETS150/3, and I love using it!
HTH