A very difficult question to answer,see banks,gold and bitcoin all have their advantages,investing in bank gives you interest which is low and it is a long term process,but your money is quite safe there,Same thing happen with gold,due to its physical presence and it being a valuable element which is limited in source ,its value will remain good in future ,bitcoin on other hand is having great scope to grow further,seeing its increase in recent times,its expected that it grows same in future so investing in it will be great choice.
But i still recommend not to invest all in bitcoin because as being online currency at has its own flaws,save in banks and invest in gold too..make a balance of saving and investing at right places.
Just because the money in banks are reserved in lockers in physical form, does not mean that it is all safe. We live in an evil world. Banks have been hacked in the past and the amount is in billions dollars collectively. So bitcoin is safer because it has strong algorithm and as far as the holder shows a little care, it is impossible for hackers to pass sha-256 hash. This is some powerful hash. Invest in bitcoin, this is the best.