There is no cap on the number of Etherium that can be mined. Mining may or may not stop once they go to proof of stake.
Bitcoin with a fixed cap has a controlled rate of inflation. Which causes a deflationary effect. By that less of your bitcoin should buy more.
No coin is really useful as a currency. Until it has reached a large enough of a market cap. So that it can handle a million transactions a minute and not have the value of that coin change mire than a few percent.
Hope I languages this well for you.