If, by chance, you have a Bank Of America debit/ATM card you can bypass the 3% fee at ScotiaBank ATM's. I seem to recall one near the casino in Maho and I use the one at the Westin on Dawn beach. Really saves on the amount of cash to bring.
As far as currency, use dollars. Readily accepted on dutch side and some French establishments give 1 -to- one. I have found that some of my cc's will charge a conversion fee not only on French side but on dutch side too. Not car rental but at grocery stores.