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.
Actually in that kind of case, it is not only bitcoin that can actually give a huge amount in profit as there are some altcoins that can do that, but when it comes to sustainability and not just some pump and dump in which price is pumped today and it is back to the historical bottom tomorrow, then bitcoin is for sure the best to look forward to investing in for the long term. Some altcoins may still fall into this category based on the support they have, but they are quite few.