I will suggest you to buy only BTC and ETH in current market situation. Market is very unstable currently and thus there is risk with alt coins.
Yes we can earn more in future for investing on bitcoin, as bitcoin is cheap and profitable for future now a days, but choosing ETH depends on your own desire but for me its probably better to invest money now on bitcoin, the world of crypto market is going up, it will raise high this year, so at the end of 2018 bitcoin will bring even greater profit.