Hmn.. lemme see.. A digital grand piano is not portable, whereas a keyboard is portable. In terms of the sound and touch response, a digital piano has much better sound than keyboard. Keyboard weight/touch, both are lightweight/soft touch. MIDI port, i think all digital pianos have standard MIDI port, whereas only the more expensive keyboards have them. Digital piano has built-in pedal, while with keyboard you need to purchase it separately. (I suppose you know the importance of the pedal). Choices of built-in/programmed instruments, the more expensive keyboards do have more instruments to choose from than a digital piano.

I believe it is the sound that makes the digital piano expensive. Which brand to buy, digital piano or keyboard, regardless of the price, i think that should depend upon your level of playing.

Good brands that i know are Yamaha (very nice set of sound of programmed/built-in instruments, both on digital piano and electronic keyboard), Kawai (i'm not a fan of Kawai actually, but i've tried a Kawai digital grand piano, sounds like Kawai acoustic piano and that's great news for one who cannot afford an acoustic piano), and Roland (love that cool and mellow sound).