Well, that's a good question.
I presonally prefer the original Furby over the Emototronic Furbys, they just seem to have more "personality".
I wasn't very excited about how quiet and untalkative the Emototronics are, though that might be a strong positive for someone else. XD
The Emototronics also seem to be lacking in the interactivity department. You can only give them a small amount of different commands/questions, to which they will react to in pretty much the exact same way each time. (or perhaps mine is just weird that way. XD)
But a cute feature is that sometimes the Emototronic Furbys will refuse a command, become sad or disinterested. XD
Here's a basic look at what they can do:
Furby (1999)
Sensors: Tummy, tongue (for feeding), back (petting), tilt (movement), light, sound.
Movement: Ears, beak, eyes, base (for dancing).
Vocabulary: Approx 200 words.
Can interact with: Furby, Baby, Furby, Shelby, Furby Friends.
On/off Switch: No, but later generations can be made deep-sleep with a specific action.
A LOT of different colours and patterns to choose from.
Baby Furby (1999)
Sensors: Tummy, tongue (for feeding), back (petting), tilt (movement), light, sound.
Movement: Ears, beak, eyes.
Vocabulary: 200+ words.
Can interact with: Furby, Baby, Furby, Shelby, Furby Friends.
On/off Switch: No, but deep-sleep is available with a specific action.
Many mostly pastel colours to choose from.
Emototronic Furby (2005)
Sensors: Tummy, tongue (for feeding), back (petting), tilt (movement), sound (voice command).
Movement: Ears, beak, eyes, base (dancing).
Can interact with: Emototronic Furby.
On/off Switch: Yes.
Limited colours to choose from.