I have a budget of 1500$. I've never played piano before. Should I buy a digital or acoustic (maybe used) piano with the budget I have? Does a 1500$ acoustic piano sound better than a digital one?
So many factors involved here.
Where you live, where the piano is situated will have a bearing on how an acoustic piano will cope with changing temperature/humidity. No such problems with an electronic (digital) piano.
Unless you pay a lot of money, you won't buy a good acoustic piano - which will need transport and re-tuning soon after - an extra expense. And every year or so after! And, will take up much more of your room - and probably not be as quiet for practice, as an electronic one, with headphones.
So, as a beginner, a fairly priced electronic will not be detrimental to your touch, will be better value for money, take up less room, be easier on your neighbours, will provide extra facilities (should you need them), cost nothing in maintenance, probably be easier to sell on, consequently, would be a better bet initially.
After a year or two, an acoustic may well be a better replacement. However - buying an acoustic, whether new and especially pre-loved, is a more involved job than buying an electronic. Take an expert along at that time.
With an acoustic, there'll be a better action but essentially one sound. Bear in mind that actions vary from piano to piano, so merely buying an acoustic won't necessarily mean it has a good action, or even one that suits you (which you won't really be aware of for a while after playing anything!). Electronics are getting better with action - but for many 1000s. Their advantage is a plethora of sounds, a lot of which sound very good. And there's little maintenance. And you can record on them. And they often have built in drum patterns.
I use both, and am lucky enough to have both. But if one had to go: difficult decision. It also depends on what style of music you wish to play.
If you just want it for joy and hobby you could buy a digital piano. (There are many advantages: Price, costs for tuning, weight, transport, loudness, mobility, other sounds, connection to computer and others. If you have the goal to become a pianist (level of piano teacher) you need an acoustic piano.