I've found digital pianos to be most helpful in the development of technique. If you turn the volume down low, it forces you to play harder to hear the notes, thus strengthening your fingers(old heavy steinways have this effect as well, but those run in the millions of dollars sometimes). Then if you go to a quality acoustic, you can refine your touch and dynamics. Although this approach may not be right for everyone, as I had to have the Clavinova repaired many times, and since the keys are plastic if you sweat at all they get oh-so-annoyingly slippery. But there is really no point in upgrading to a grand if all you can afford is a crummy one. It's much better to either buy a good upright or wait untill you can buy a nice grand, or ask to use the facilities at a nearby college or institution, if they have music programs they most likely have practice rooms with quality pianos (steinways and upscale yamahas) that are kept in tune.