Your first Festool should definitely be the dust collector - a CT26 or CT36. From my own experiences, sanding without a good vac is miserable. Sanding with a good vac is a great experience.
Presumably you're interested in the Festool air sanders because of the shape. With that in mind, you may want to take a look at the Mirka Ceros. It's a DC electric sander that handles like an air sander. Assuming that they have not changed the OEM pad, you can use all of the Festool paper and the Abranet paper on them. And the Festool D27 hose can be screwed into the Mirka's dust port. Dust collection is excellent. I have the 6" model and it's my go-to sander for general sanding. Search for Mirka Ceros on the web. Be warned though, while it's Festool quality, it's also Festool price.
My second go-to sander is the Festool RO90. I don't make furniture, but with its multitude of pads, I would think the RO90 would be a great tool for furniture. Dust collection is great.
I have five Festool sanders: RO150, RO90, RAS115, RTS400, and LS130. Each has it's purpose. I'm thinking of getting an ETS 150/3 for finish sanding. With the possible exception of the Mirka Ceros, Festool makes that best sanders on the planet. (In my ever-so-humble opinion, of course.)
Dan.