I'd say the Deltex.
To understand this, you got to look at the nature of the two machines. The DTS400 is more of a pure finishing sander. It has a 2mm stroke and the pad is directly connected to the motor. It also has a bigger pad so the force of the motor will be spread out over a bigger area. This makes it less aggressive but leaves a finer result. The Deltex on the other hand has a slightly bigger stroke, 2.4 mm, and the motor is connected to the pad through an angular gear mechanism. This makes it a stronger and more aggressive sander. And since the pad is also about 40% the size of the DTS400's pad, that force is concentrated on a smaller area.
So if you're sanding on rough surfaces, like a floor, or that of an industrial machine like a tractor, the Deltex will do much better than the DTS400. Both floors and tractors don't require the very fine surface quality needed on exclusive hand made furniture like a lot of the woodworking guys here make. In the last case, the DTS400 would be better overall.
When working on the tractor, the Deltex would also reach into tighter corners than the DTS400 because it has a lower profile.
Now if you had no sander and the Deltex or the DTS would be your first, I'd say the DTS400 since it will allow you to do both detail work and larger surfaces. But since you want a sander to compliment your Rotex, I'd definitely go for the Deltex. Use the Rotex for the large surface work and the Deltex to get in all the corners.
To be honest, I had the Rotex and wanted to get me a DTS as second sander, but I got me a deal on a brand new Deltex that was to good to walk away from (only 60% of standard price). Been using it for a month now on house trimming and I am very pleased by its overall performance. With 40 grit paper, it cuts through thick paint like it's butter. And with 120 grit or higher, you leave a very nice finish. Dust collection is excellent. Only thing I'm less pleased with is the on/off switch which I think is too big and placed wrongly. I keep turning it off by accident when reaching.