I'd agree that we need to know the surface sizes you will commonly be sanding. For large panels, casework and "grinding" rough surfaces, the RO150 will be faster. For face frames, rails & stiles and smaller pieces, the RO 125 will be easier to handle.
I have the RO125FEQ which has been a stellar sander. Also keep in mind that if you want to follow with a finish sander, (i.e. ETS125 or ETS150/3 or ETS150/5) you may want to match the pad size so your sandpapers will be interchangable.
Don't forget to get a dust extractor to go with it in a package deal! The Festool sanders truely excell when used in combination with the dust extractors. For sanding alone, a mini or midi will be sufficient. If you plan on more tools in the future (gulp), like a saw or router, go with the CT26 or 36.
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