The ETS150 sanders are one trick ponies. However, they do that one task better than any sander ever made, IMO. There are simply the best finish sanders you can buy, period. The RO sanders are great sanders, however they are not as refined as the ETS sanders. They will remove far more material and do it with ease, but I find them to be cumbersome when it comes to finishing tasks. Compared to the ETS sanders, they are much heavier and have a lot more vibration to them. They are great for certain tasks, but not the best sander for everyday tasks, IMO.
I agree with what Eli said, post a little about what your intended usage will be. You'll get much more specific answers that are tailored to what you're looking for. There are a lot of us here that own both and are quite happy with them. What we are actually doing with them determines which one we will use though. Both excel at different things.