When investing, Im a big fan of diversification of portfolios. I believe that both Ethereum and Bitcoin offer value, albeit in different ways.
Even though Ethereum is positioning itself to become a large platform, the fact that Bitcoin is so well known makes it the first place where people invest.
The answer is already in your post. If you are a fan of diversification of portfolios, then it is advisable to invest in some other alts other than what you currently have now and Bitcoin. ETH is good. You can add it to your portfolio list. Diversification has always been the advice of experienced traders and holders.