Bitcoin and Etehreum both are best for the long run. You can buy both of them. Ethereum is very undervalued right now, Most people believe Ethereum will be hit a new all-time high price within a short time. My target is $8k to $10k and I believe it will happen within one 1year. There is no doubt that bitcoin is the best coin for long-term investment. You choose both good coins. Don't hold all of your coins in only one wallet. Try to use some other wallet also, I suggest you use MEW for Ethereum(ERC20) coin.