I am in Cambodia for a month, and, while I brought enough cash for everyday expenses, I am thinking about making a larger purchase (a bicycle) for which I would have to use my Krungthai standard master debit card. Gemini says that visa/master charge a 1% fee, while Thai banks usually charge a 2.5% fee, which would add up to 3.5%.

To test it I made a smaller purchase at a supermarket, did the math, and it turns out that I lost 2.3% compared the XE midrate of the day.

The question is, would I lose more (or less) if I withdrew the cash I need at a Cambodian ATM and pay in cash, or to pay with my Thai card at the shop?

Should be the same