The best value for the money is the Digitech Vocalist VR. It's a single space rackmount with digital reverb that is accesable from an external effects bus. (very cool)
The advantage over the Korg is the number of voices it plays at once and the effects.
The advantage over ALL the built in keyboards is the sound. It's just better. I don't care WHO makes the chip - the Digitech sounds more relistic than the Korg, Yamaha and Ketron units.
The TC is great, but costs a lot more, and it shuts down when you change programs ... the Digi does not. Makes smoother changes within a song.
Tom - the Midi Vocalist is an older technology and is more synthetic sounding. The tabletop model "workstation" has the newer sounds, and has more real time control on the unit.