I have experience with both of these sanders. The DTS is a better finish sander, there is no question about that. However, it does not excel at moderately tough sanding tasks, and it is outright terrible at aggressive work. The issue is the tip of the DTS sandpaper wears out quickly, and can't be rotated since, its not a triangle. Worse, it's fairly easy damage (read: ruin) the tip of the sanding pad doing moderate/aggressive work. That will cost ~30 bucks. Personally, I'd pass on the DTS if you want a sander that can be used for more than finish sanding. If finishing sanding is your thing, the DTS gets into corners well, and the dust collection for finish work is very good.
The RO90 is an interesting sander, and one I've used a fair amount. Since you already have the RO125 you know there is a bit of a learning curve with these sanders. Don't be surprised if you don't love the RO90 at first. Its a fairly powerful sander in a small package so it can be a handful. Trying to muscle it will likely make controlling it harder. It takes a light touch and once you learn that the sander can be a real gem.
As a delta sander, it is one of, if not the best I've used. The paper and sanding pad are triangles, so they can be rotated when you get wear one of the tips. This allows you to be rough on the paper/pad and not break the bank. The RO90 also has a larger diameter orbit/sanding stroke so it sands faster than the DTS. While the RO90 can be used as a finish sander, it doesn't excel in that role. You'd be much farther ahead to break out your EST125 for finish work. Dust collection for finish sanding is okay, for aggressive work the collection isn't great.
One of the nice things about the RO90 is the ability to quickly change between the round and delta pads. I spent a lot of time using the RO90 on a ladder/stage/scaffolding doing exterior repair work. I could have one pad on the sander and the other in my pocket or toolbelt, and I'd be set for whatever sanding I needed to do. You don't need to be on a ladder to appreciate that. Having one sander/cord/vac hose is a lot easier than dealing with two.
The RO90 isn't cheap, but I found it to be one of my most used Festool sanders (with the ETS125 next). I found the RO90 more or less replaced the need for my RO125. If you go this route, buy extra delta pads, and get the hard round sanding pad. Also, this sander runs hot, especially when new. So if it feels hot in your hands, that is normal. If you have any specific questions I'll be happy to try and answer them. Best of luck.