Just the size and weight are different than one might suspect from the photos and information here. The large unit brings almost 9 kilos, the small, only about 3 - an important point, especially for bass. The size is significantly different, the ST20 is small and handy, the ST30 requires even more space on the shelf or on the TV rack. Attention, which are surrounded at Amazon Weights and Measures (at least currently) completely wrong, here the correct dimensions as on the Bose site on nachlesbar:
SoundTouch 20: Height: 18.8 cm, width: 31.4 cm, depth: 10.4 cm, weight: 3.2 kg
SoundTouch 30: Height: 24.7 cm, width: 43.5 cm, depth: 18.1 cm, weight: 8.39 kg
In both devices, the sound is - just as one would expect from Bose - outstanding. Any settings way, there is not - only louder and quieter, no control of sound mixing. Even the ST20 enough to fill a room of 30 square meters with rich sound. One wonders, what comes out of such a small box. But if you listen to both devices in direct comparison, the ST30 has not only volume, but also the significantly richer bass and a little clearer highs and mids. The greater weight pays off, and the higher price for the ST30 has quite a value. Considered Sonically I'd invest the money for the ST30 perfectly.
But I've returned both units again. And this is due to the (still?) Totally inadequate software. The registration and configuration process on the PC is somewhat cumbersome and tedious, but that would be acceptable, which makes it so only once. The main shortcoming of Sound Touch is that it then just the music library of the computer plays, of which it has been configured. If this computer is off, there is no access to the music library! The device does not support DLNA server, so you can not access any kind of centralized media servers that are located in the home network. In my household, it is, however, so that the media does not stack on each and every workstation, but could usefully be collected on a central NAS to access all workstations from which it then. This is a useful principle which underlies the DLNA standard, which is supported by an increasing number of manufacturers. Bose has apparently not (yet) understood this fundamental principle of home entertainment in the digital age. Too Bad. Thus, the expensive equipment is not useful for me.
One can only hope that it is because the sound-touch devices are still quite fresh on the market and therefore the software is not yet mature. It would be hoped that Bose will subsequently supply a full DLNA support here. Then I would think, zuzulegen such a device again. Until then my advice: Stay away from it.