Both are very good for invest, the only different is the price, not everyone can reach bitcoin price, but on the other hand, most of us can reach eth price, so for me i choose eth. But if you have a money to invest, you can split your money into btc or eth.
I also think that it is better to divide the portfolio into several parts. In my opinion, the portfolio should store various tokens, as well as ETH and bitcoin. And the number of ETH and BTC should be more than 70%. For the remaining 30 percent you can buy altcoins.