Unless you can actually get a DX 93( it's been discontinued in many places), then your current choice from Festool is the RO90 and the DTS400. They're so different that your needs should make choosing between them easy.
However, here are some of my thoughts on each sander.
DX-93, It won't be a one handed sander for the most part due mainly to its long body. It's better to lightly keep the pad down and flat to prevent wearing out just the outer tip of abrasive that you're using and heating up the Velcro style pad as well. To do that, you usually use two hands, but it's not a struggle with this light sander. You can use it one handed, but watch your pad angle in the front while you're using it.
Not that quiet of a sander either for its size and type, has its own high pitched noise when it's running, but it sneaks into many tight spots for me and is smaller than a RO90 for the same tasks of sanding, so I tend to reach for it over the RO often.
DTS400, larger pad size, easy to keep it flat on a surface and very easy to use one handed. Great finishing sander for wood once you dial in your speed versus vacuum suction levels for the grit abrasive you're using. I use it for paint work or drywall compound too. Downside is the lack of turning or rotating abrasive sheet for wear since the pad shape isn't symmetrical like the RO90 or DX93.
RO90, you have the 3 modes, you could own just this sander and be really happy with that ability since it's in a class of its own doing that. Two handed operation for the most part, I have Delta sanded with one hand since the sander isn't hard to control in either Random Orbit or Delta mode, but the balance of the tool doesn't really lend itself to one handed usage from what I've seen working with it. You also get the ability to rotate that small abrasive sheet for wear while you're sanding. Dust Collection in Delta mode is weaker or less affective for this sander compared to using it in the other two sanding modes. This is due to the Delta head attachment moving the dust pick-up further away once it's installed. Compared to the earlier DX93, the RO90 won't capture as much but in some cases like outdoor work that won't matter to you.
