Thanks, Nick,
The RO-90 in delta mode has a larger stroke than the DTS-400. 3mm vs 2mm so you should be able to finish faster. The larger stroke of the RO-90 seems to make it more aggressive, but it also has a smaller delta head than the DTS-400.
Both have eccentric motion so the pattern should be about the same. One of the videos had the two in delta mode being pulled across a previously finished surface. The scratches are a bit apparent there, but they are also at a low grit so expected. In comparison, since both are eccentric, the pattern should be about the same. It isn't a window-wiper motion so you don't have pronounced repeated scratches to deal with.
For fast stock removal, the RO-90 is a natural whereas the DTS-400 shouldn't be considered.
I'll say this, the DTS-400 was one of my favorite ROS sanders and is still up there for the large flat pad (nb, I don't have any ETS sanders); it worked well on rails/stiles. The RO-90's versatility is why I reach for it pretty often and it seems a natural for rails/stiles.
I presume you are comparing these two for the delta action; since the round head can take care of the rest of the surface pretty quickly and you switch to delta for the corners, my hunch is the RO-90 would be faster.