Credit card/Cash

Going in a few weeks to some Pacific countries, among them Samoa.
Can anybody here recommend taking credit card or US cash as main supply?
I am mainly thinking about any difference concering the rate and charges.
And what credit card, is Master card OK?

I am aware that I will need to stock up on local currency for distant islands.
Regards, Erik

What was said in regard for Vanuatu is right for Samoa as well and most likely for all island countries on your list. You need the local currency.
In Samoa, US Dollar cash is sometimes accepted for payment but always at a bad rate. Bank cards (Maestro, Cirrus) are accepted at ATMs as well as VISA and Mastercard. The ATM rates are usually better than the cash rates at the banks. Prefer Western Union outlets when changing cash as they give a better rate.
Hotels, car rentals and some stores in Apia take cards - VISA is a lot more common than MC though. No credit cards at gas stations and village stores on any island in Samoa! Get Tala.