I`ve tried both. I`m a painter so I use deltas a lot, mostly vertically flattening paint on windows and woodwork. I also need to be one hand only and so the DX is perfect for me. The RO90 is 2 handed operation and I find it a bit unbalanced, a bit cumbersome to continually change direction and angle. I enjoyed the power to take off paint with the rotary action but that`s only with a small area. Mine is going on ebay and I`ll get a RO125 instead (and keep the DX).
I`m guessing that for you woodworkers the 90 much more useful as most of your work is horizontal on a bench.
Getting off topic in my first thread!