I do not believe ethereum can rival the existence of bitcoin because the difference in exchange rates is very far made bitcoin will still be popular as an investment place that is often used by traders.In creating income every time.
And the number of circulating supply or total supply is different aswell, in this case a slight price increase in ethereum will definitely give huge impact to the total market capitalisation.
Consider that, logically a coin that have huge amount of supply will definitely have less price than a limited coin like bitcoin that only have 21 mil.