For me the answer is obvious. Bitcoin is a more reliable value. This is happening even now. In the long run it will be even more pronounced. The mechanism of inflation underlying the Ethereum will never allow him to become promising in the long term. He's probably already reached my limit.
Well I guess it is better for you to invest in both Bitcoin and Ethereum because cryptowolrd is unpredictable we don't know what might happen before or after 5 years. However, for me investing on bitcoin will give more profit because compared to other cryptocoins bitcoin is more fluctuative compared to them.