They're really different beasts:
Model One: single speaker
Model 88: two speakers, but very close together
Model Two: two separate speakers
Something to keep in mind is that all of the "stereo" table radios (Model 88, Bose, etc.) really struggle to present a true stereo image because the two speakers are so close together. If you're right next to them, maybe, but from across the room, forget it
I compared them when we were shopping for table radios, and found the Model One and Model 88 to sound closer to each other than to the Model Two, mainly for the reason mentioned above. Both sounded more like "mono" radios than true stereo ones. The Model One had a slightly more pronounced midrange and upper bass than the Model 88, but overall had a more enjoyable sound to me. But the Model One also had a much better tuner and much better build quality and appearance (IMO). I just like the real wood enclosure, and the 5:1 geared tuning knob is the best tuning knob I've used (sounds cheezy, I know).
The Model Two is what we ended up buying, basically because you get two separate speaker units, so it's possible to get true stereo sound. The sonic signature of the Model Two is still a bit emphasized in the midrange and upper bass, but it's very non-fatiguing and enjoyable. It's possible to have it on for hours without being irritated by the sound -- a trait few small radios have.
The other things I like about the Model Two (over the Model One) are that it has an line-level auxilary input, a mixed input (which gets played at the same time as the radio/aux), a line-out, and a stereo headphone jack. It also has the best wood/finish combo of all the Tivoli's IMO.
Hope this helps!
BTW, here are all the Tivoli's at once:
