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Well the advantage to Monzo is if you speculated the wrong way with Revolut then you wouldn't have lost money with Revolut. And the disadvantage is that if you speculated the right way then you would lose with Monzo.
Revolut's selling point is holding multiple currencies at the same time.
Monzo just applies the exchange rate of the moment with no extra fees.But I don't think the intention is really that people are using it to swap money a few months before a holiday. I'd guess it's more that if if you're working for a while in the US, or being paid in dollars then you can work and spend in the same currency rather than having to think about everything divided by 1.41. No wait, 1.39. Oh, 1.42 today.
You can do either with Revolut, you can change money to Euros or Dollars in your account whenever you want or you can just keep all the money in sterling and it will apply the rate at the point when you withdraw the cash.
Personally I opt for the latter as trying to play the fx markets is clearly going to go wrong.