I haven't studied the high-end models, but basically I think the abilities grow with additions of
*pronounce street names
*acts as bluetooth phone extension
*route based on traffic data
*mp3 player

They also vary the price within a model range based on the maps that are loaded.
In the 'old' 200 series, the 200 was the cheapest, with only USA maps loaded, then the 250 with all of NA, then the 270 with NA and Europe. All of these were the same hardware and software.
The 200 has been superseded by the 205, so I imagine you'll have the map-database extensions being 255 and 275.

Like I said, I haven't studied the upper level stuff or the new stuff- the basic unit is good enough for me. Plenty of info out there, though.