why btc only? etherium is also good to hold and invest due to the fact that it is more stable and still increasing overtime, sooner or later eth can possibly reach on what bitcoin has achieved because eth is indeed a good coin with lots of potential to succeed in the near future. you dont have to wait for eth to go down on that specific amount because eth is verry cheap and afordable for all the time , even if you buy it in one whole you can still be able to afford it unlike to bitcoin that is pretty expensive right now.
However I support diversification more than investing in only one asset. This increases the number of sources and decreases the chances of loss.