I agree. I remember using the una-corda on the Sauter and feeling that it was a different voice, not just softer or muted. And when I played the Sauter, it required more "weight", not "work". 
The Shigeru was almost too easy to play, I remember the action being shallow compared to the Sauter. But the tone was very, very beautiful. But not like the Sauter, which seemed to have three (or four) dimensions to it's tone. Oh which one was cheaper? (for your location) I don't recall...
Almost too easy to play..that was the word for it - effortless. The Shigeru's action was definitely shallower than the standard Sauter action, but mine being the accelerated action, feels around as shallow as the Shigeru, though still slightly heavier than the standard action even. In any case, key depth can be easily adjusted by a competent tech.
Another difference I forgot to mention between the Shigeru and the Sauter, was the Sauter's more singing treble line. The Shigerus I encountered generally had a big, warm tone, with full of complex overtones surrounding the fundamntal tone. Treble was generally a little subdued relative to the rest of the range, and I detected a "dead octave" above the middle C, where for some strange reason, the notes refused to stand out. On the Sauter however, the treble was distinct, singing and could easily penetrate through the bass line. Also the Sauter sound was generally clearer and more penetrating, with less overtones, though the fundamental tone was still rich and warm, unlike say, a Schimmel, which I find clear, but thin.
The Shigeru, while powerful (especially in the bass), had its power more "in your face", unlike the Sauter, whose power is more subtle, less "in your face", and relies more on its great depth of sound. THis "depth" actually gives the illusion of a piano that is bigger than it really is. I must also say that with the new accelerated action my Delta is capable of the most subtle shades of tone I can imagine - definitely a wider tonal palette than the Shigeru.
That of course means the Sauter retails considerably more than (up to twice) than a Shigeru of comparable size. Inch for inch, the Shigeru is good value for money, but I do think if one must have that level of refinement, both aesthetically and in terms of performance, then the Sauter would make an even better choice (considering that it is still considerably cheaper than a Steinway "A" or a Bluthner/Bechstein, for example)
Topic: Photos of my Sauter Delta