I don't really see these two tokens as different or rivals because they are really good when it comes in investment. Cardano (ADA) just arrived at the cryptocurrency later than ETH but we know that the potential of it is very high and it is very recommended in long term investment, I suggest if you are going to buy you should split it into two because in the future these two tokens will dominate.
ETH improvements are faster than I thought so I also have some of these tokens.