Welcome to the Forum and the Festool world.
I am going to buck the trend here and advise you look at a larger RO sander if that is the way you are leaning. I have a RO 90 and like it, but I also have a RO 150 and use it about 4 times as much. The RO 90 is good for small projects and sanding narrow pieces, but it you have large panel projects it will take you forever to complete the job.
If you are only planning to stick with Festool sanders, then a Midi or Mini will work just fine, but if you decide to expand your Festool inventory to a track saw, router or domino, you should look at a CT26 or CT36. I have a CT26 and if I knew then what I know now I would have gotten the CT36 for additional capacity. Of course this is just my opinion.
Best of luck with you choices. I know you will be happy with Festool in whatever model you choose.