Nothing is a 100% safe including banks, bitcoins, and the building you are living in.
From an objective point of view, bitcoins seem riskier than gold. The instability of it is dangerous even if it can rise to levels. There can be better altcoins than bitcoin that would go to the moon.
The possibility could happen but with bitcoins it is already tested through times and it's the father of all the altcoins around so the any movement of the bitcoin would certainly affect the altcoins, on the other hand in terms of investment I agree that gold are safer but that is form long term investment while bitcoins you can maximize your earnings if you are good in trading.
I would say the best investment is neither but land and property, as we can see, Gold has been in a consistent decline from its peak of $1800s and it doesnt look like it will end here with banks issuing gold paper bonds and the likes. Bitcoin comes after Property in terms of investment growth, however for stability i agree gold or property are the safest.
It also depends but if you treat bitcoins as an investment, for me it has more room to grow your investment that just buying a property that would subject for depreciation and if you can notice bitcoins is still at its early stage so we don't know yet what price would it peak. I really find investing in bitcoins is a profitable way yet more riskier but I am willing to take more risk for more profit.