So it seems Visa is transparent on their rates...
https://www.visa.com.ar/soporte/consumer/viajes/tipo-de-cambio-english.html (apologies if this link has been posted already)
And as has been noted, you can check your actual Visa rate in real time here:
https://usa.visa.com/support/consumer/travel-support/exchange-rate-calculator.html
I've now done this with 3 purchases and the amount charged to my Visa debit card (in my case in AUD) is correct to the cent. Each appeared on my online bank statement immediately. Must admit I was pleasantly surprised.
My question is which gives the better rate - Visa or Mastercard - or are they identical? Given that Mastercard operates differently to Visa applying refunds after some days, has anyone compared the rates for both? I'm inclined to just use Visa as it's instant and transparent, unless Mastercard is giving a demonstrably best rate after refunding? Appreciate any and all insight!