There is no "better" or "worse". The S&P500 is less volatile than Bitcoin, the flip side of that is that historically Bitcoin has appreciated relative to the S&P 500.
Bitcoins are far more practical than an S&P500 ETF. For instance, I can carry bitcoins across a border, whereas if you want to move shares across a border it will involve a lot of paperwork -- if you are a US citizen it may be impossible due to FATCA.