BigPay card is the best if you are performing foreign currency transaction, AMEX charge is the highest among all, i always use BigPay card whenever I am overseas .

If you have nothing else to spend till end of the month, then use your M2 Amex Gold/platinum for TWD7k (near RM1k).
But if you have HLB cashback card, you may consider the 10% CB promo.

Depending on what you want.
Of course by Amex you need to pay higher, but you earn TP.
If you are targeting lower rate, got with Bigpay.
Difference by around 3%.

I can't really find exchange rate for Amex, but previously my calculation was based on Visa rate + 2.5% (bank charges).
Bigpay is based on Mastercard rate, which is slightly lower than Visa.
For now, there is no extra charges from Bigpay (0%), solely based on MC rate.

It's 2.7% from all the cards I've seen. If the American Express card you're using provides purchase protection, and you'd like some insurance against theft or damage (or the ability to file a chargeback in case of fraud), it's a good idea.