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.
Bitcoin is the only coin that would give you a much bigger profit than most of the top coins without a doubt, but there are some coins with small valuations that could give a much bigger profit in a short period of time, there is no specific time to invest in these coins if you are planning to hold for the long term, but for the short term you need to monitor the news and market situation before investing.