Wow, so many different replies, thank you.
One of the sanders I'll keep is the RTS 400. I bought that to use instead of the RO90 on face frames and I have the hand sander with the same size paper. I'm think it will also serve to replace the RS2E I just sold, just taking longer to sand the same area.
I like the RO90 a lot, except it can be fatiguing when holding it one-handed and moving around face frames. I do find myself turning to it for this odd sanding jobs, most recently a staircase project. I still have a bunch of paper for it.
I bought the ETS EC 150 to replace a ETS 150 I sold that was my first Festool sander. The ETS 150 was really good, but the EC version is that much better. I still have quite a bit of the older Rubin paper without enough holes left over I want to use up before ordering newer paper.
I bought the RO150 mostly because I liked both the RO90 and the ETS 150 and hoped I'd reach for it more than I do. I don't use it much but figure I'd really wish I had it when it would be needed. Also, it will share the paper with the ETS EC 150.
So, if I kept the RTS 400 and the ETS EC 150, which of the RO sanders would you sell?