Digital vs Acoustic pianos
Digital Piano Advantages
Digital pianos usually have an array of features that make them an attractive alternative to an acoustic piano, such as:
Different types of piano sounds
Other keyboard sounds like harpsichord, organ, etc.
Other instrument sounds like strings, flute, percussion etc.
Built-in rhythm capabilities to accompany your playing
The ability to record your performance
The ability to interact with other electronic music devices (MIDI)
Never a need for tuning
Headphones to practice in private
Digital Piano Disadvantages
The problem with digital pianos is that they can't really duplicate the tone and touch of a real piano. Even with today's sampling technology individual notes may be quite accurately reproduced, but the tone of notes sounding together, as in an acoustic piano - with complex harmonics mixing and resonating against a flexible wooden soundboard - cannot be matched. As a result, most music sounds rather sterile played on a digital piano.
Digital pianos don't feel like real pianos. "Touch Sensitivity" and "Weighted Action" is not the same thing as the sophisticated inner mechanism, or "action", of an acoustic piano. Digital pianos merely simulate the touch of pianos. They don't provide the same feedback or responsiveness to your playing, so your performance range is limited.
If you are considering buying a piano for your child, consider this: many piano teachers will not teach students who have reached a certain level on anything other than an acoustic piano. Digital pianos are counter-productive when it comes to technique and dynamic performance. These skills cannot be practised on a digital keyboard and then applied to a real piano action. It's not the same thing. A piano also represents a stronger commitment to a student, as opposed to a digital keyboard that may represent the same thing to a child as a computer, an electronic game-station or a CD player.
Something else to consider is that an acoustic piano will hold its value far better than a digital. An acoustic piano can last 100 years, while a digital may be obsolete in 5 years and might be hard to even give away by then.
Hybrid Digital/Acoustic Pianos
There are hybrid digital/acoustic pianos on the market, like the Kawai Anytime and Yamaha Disklavier that combine the features of both. These are regular acoustic pianos with real piano actions that can be disconnected with the digital sound on demand. Further, they are MIDI capable. These pianos are quite expensive and out of the price range of most people. If, however, you can afford one, these pianos are the ultimate in a total piano experience. They are the new generation of pianos: they can record and play back your perfomance with deadly accuracy; can play a wide selection of pre-recorded music of all styles; can be used in conjunction with piano instruction software; and can be used as a MIDI controller to trigger other electronic tone generating devices like samplers, synthesizers, drum machines or anything with a MIDI interface.
If you are looking for a keyboard that can be easily transported, or connected to a sound system, or one that can be used to record music using computer software, or has the ability to use headphones for privacy, then a digital piano is the way to go. Otherwise, think very carefully before you decide to buy a digital in place of an acoustic instrument.