I've been a musician (primarily voice, but I play an assortment of different instruments, including piano/keyboard) all my life, the son of a piano teacher with both a grand and a vertical in our home. I learned to tune pianos as a senior in high school, now going on 28 years ago. I've been the concert tuner/technician at two major universities, for dealers of Steinways, Yamahas, Baldwins, Kawais, Samicks, Young Changs, etc., and I've owned both acoustic and digital pianos. I am now a dealer of both acoustic and digital pianos, and I continue to be a piano tuner/technician very much in demand.
From my perspective as a musician, technician, and a dealer, the one thing any acoustic piano will always be able to do that a digital will not ever be able to match is touch response. If your goal is to learn to play an acoustic piano and be able to control the infinite degree of touch response that only an acoustic piano is capable of, then don't resort to the digital. An acoustic piano produces not only an infinite range of dynamics (degrees of loud or soft), but also an infinite palate of tonal color as well. A digital limits you to (typically 5 or 6) levels of loud or soft - far from infinite - but tonal changes don't happen at all with a digital. With the digital, you get the same tone, be it soft, medium soft, medium, loud and a little louder. While keyboard weighting has been an excellent improvement for digitals, we will never be able to replicate the actual effect that the system of levers inside the action has on that actual hammer to string contact - be it in the vertical or grand action (not to mention the many variations in hammer density, scaling, etc.) With the piano samples alone, there is no contest between the digital and the acoustic piano (assuming we're not talking about a junk acoustic, here). The acoustic wins without a challenge. The digital simply cannot match the musical sensation of a real piano. The draw of a digital for me, lies in its ability to produce many different sounds, and/or to be able to practice through headphones without disturbing others around me.
My personal reason for owning a digital (in addition to an acoustic) is in my own musical interests, which the piano alone cannot produce, and in inspiring the interest of my children in music itself. They'll fiddle around with all the sounds on the digital and start actually composing music a long time before you can get them to sit down and learn music on the piano. But at the point where they need to start developing skill with playing music composed for the piano, I'm going to push to steer them to the acoustic. And if I'm accompanying myself to sing with a keyboard of some type, if I want just the piano sound, then the digital will be my second choice every time. If I want a combination of sounds, then the digital will be my first choice. But I can get by with a well-maintained, even if the tuning isn't clean, acoustic over a digital any day of the week.
So, to me, the choice is not whether to change from vertical to digital, but why not both?
That said, I am very impressed with the offering of features in this new line of digitals from Samick. For the price, I don't think anything else on the market comes close, and it is my understanding that dealers of Yamaha digitals are placing large orders for the Samicks. One model, the SSP-20, is so popular that it is back ordered 2 to 3 months.