QUOTE(Darren_Resonance @ Aug 13 2009, 12:25 PM)

But is the touch (like playing loud or soft) of this two types same? If I am using a digital piano to practice,and I am using a acoustic piano during exam,will there be any huge difference?
Depends what level you are really. Up to about grade 4, I don't think you'll really notice the difference, but after that the difference will become more and more apparent.
The touch of (even the really expensive) digital pianos is nowhere near as good as any half decent acoustic. You won't necessarily notice it though until you get to around grade 6+ though. The first time it really hit me was in the warm-up room for my grade 8 exam, which is basically the digital piano showroom of a shop, so it wasn't that they were bad digitals.
At home, I have both a digital (Roland FP7, which is pretty good as they go) and an upright (quite nice but nothing special). If I practise on the upright, I find that I can play them on the digital no problem, but the other way round, it's like I've hit a brick wall. (This is more apparent with faster pieces than slower ones)
The other main problem with digital pianos is the pedal, which is never realistic, even if it's not a straight on/off one. The upwards resistance in it is no where near enough and you can't feel the vibrations through it like you could with a real one. It also doesn't seem to have the effect of causing other strings in the harmonic series to resonate as well as the one you played so you don't get nearly as much resonance, which means that, sometimes, you can reasonably get away with pretty much just leaving it down throughout a piece. Again, when you go to playing on a real piano, it's like you've hit a wall and you find yourself either with a completely washed-out, over-pedalled mess and you have to pedal much more purposefully just to get the pedal to go down at all.
The only real advantage I can see to having a digital is that you can play silently. But, having said that, the noise from the keys will travel to the next room and the pedal thumping will go through the floor and is very loud coming from the ceiling in the room below, so it's not really as quiet as you'd think it would be.
Yes, it's more portable, but, as long as you only have to move it occaisionally, that's way down the list of things to consider when choosing a piano.
The hundreds and hundreds of sounds are really not necessary. If you really wanted to play the harpsichord/organ/violin/thunderstorm/train/dog

, you probably wouldn't be buying a piano. They're a bit of fun to mess around with but really not relevant if you're learning the
piano.
You can connect it to a computer to record from it, but you can record from an acoustic if you have a microphone.
Actually, the only real advantage I can think of of having a digital is that I can record recorder accompaniments sight-reading really slowly then speed them up rather than having to spend ages learning them. But that's cheating and won't exactly get you very far if you have a piano performance/exam etc or wanted to be able to actually play pieces.
So, basically, unless you have to move house every other week or something, you're probably better off with an acoustic.