ethereum is a big altcoin but it is still a pump and dump altcoin. there are so many hard believers in it around this forum but that never saves a project with so many issues. and speaking of investment, the unlimited supply of ether which already has a huge circulating supply is not going to be kind to the price in future.
this makes ether a good short term investment only during the pumps and a bad one for long term since the price will go down in long.

bitcoin on the other hand is the only cryptocurrency that has actual usage. it is adopted as a currency by many big companies such as Microsoft and lots more. even countries like Japan adopted it as a currency. that leads to more adoption and with it price rises.
and with the limited and small supply of bitcoin the price has only one way to go and that is up. which is why everyone says bitcoin is the only long term investment here.