As a piano technician and piano teacher, you can probably guess which side of the coin we fall on in this argument. However, in the spirit of objectivity, we'd like to lay out reasonable pros and cons for each side. So here goes...
SOUND
The acoustic piano definitely wins out in the sound department. There is absolutely no MIDI file or computerized sound that can replicate the wood, strings and hammers that acoustic pianos don. No digital piano can replicate the sound of felt hammers striking a piano string. Although technology has come a long way in recent history, there's just no way to duplicate the authentic sound and feel of an acoustic piano.
DESTINATION
The digital takes the prize here. If you live on the second or third floor of an apartment building, being able to move your instrument with ease (and plug in headphones to avoid noise complaints) are very important factors. However, even if you live in a two story house, moving an acoustic piano up the stairs, while challenging, is not impossible and typically only has to be done once.
COST
This is a bit of a loaded topic. While initially, it may seem more expensive to maintain an acoustic piano, the upfront cost of a digital may outweigh the long term savings on tunings. Piano teachers generally recommend a list of features necessary for digital pianos to be of benefit (even for beginner students). First, it must have 88 fully weighted keys. This feature alone starts in the $600+ price range for a digital. You'll want a good bench (usually purchased separately) and quality sound as well. Now you're looking at a minimum of $700 sticker price. If you want additional bells and whistles such as music downloads, interactive features and recording capabilities, the sky is the limit. Conversely, acoustic pianos can be found on local Craigslist or Facebook ads for FREE (although this rarely is actually free. Read more about that HERE.) You can easily find an acoustic piano in good working condition for $50 and up.
MOBILITY
The biggest challenge can be moving an acoustic piano into your home. Especially if you have stairs. You can hire a local moving company to move the piano professionally for a nominal fee (varies by location) which is often still far less than buying from a storefront or even the cost of a digital piano. Or you can find a truck or trailer, a furniture cart and 3-4 strong men to help you move the instrument. If there's a will, there's a way! Offer pizza and you can often find a few willing folks to help with the heavy lifting. A digital piano can be loaded into an SUV or truck by two able bodied individuals.
LIFE EXPECTANCY
A digital piano, like any electronic, has a limited life expectancy depending on how well cared for. Spilled liquids, pounding toddlers and power surges will stop it dead in its tracks.
The software will inevitably become outdated. Anymore, a digital piano that is more than 3-5 years old is becoming obsolete if not dilapidated. On the other hand, a well-cared for acoustic piano can become a family heirloom, being passed down generation after generation. It is not uncommon for us to work on a piece that is 100+ years old. In fact, the upright grand sitting in our living room was built in 1906 and is a beautiful piece of furniture as well as rich in sound.
FINAL THOUGHTS
As a piano teacher and technician duo, we obviously have a personal preference toward acoustic pianos. Nothing compares to the sound and quality of an acoustic piano. Once a piano is moved, the only cost is yearly tuning and occasional repairs. In our opinion, the benefit and cost effectiveness of owning an acoustic piano far outweighs the inconvenience of moving/having a large furniture piece. The only situation when we recommend a digital piano is if you are in a 2nd or 3rd story apartment with shared walls.