The coin with scarcity is the best to buy.
Scarcity certainly has an effect, but there are other factors that contribute to scarcity as well as total supply. For example, a lot of ETH might be locked up for staking.
Beyond scarcity there is obviously also use case and adoption, which are and will continue to be huge drivers of price.
But currently so much of the price is based on speculation. This should become less of a factor as crypto enters the mainstream, and as volumes increase, such that eventually the 'price' of a coin more accurately reflects its actual worth... but we shall see.