"It depends."
Usually you're going to do better in U.S. dollars, so check the U.S. dollar rate and compare it to the Singapore dollar rate. Also, make sure you're using a low cost credit card that's well suited to foreign currency spending. My current two favorites in that category are ICBC's Global Travel Mastercard and CIMB's Platinum Mastercard, in that order (although since CIMB's credit cards are annual fee free for life, you might as well get one). Do NOT use a debit card for online purchases since you get some more consumer protections with credit cards.
If the travel provider is located in another country besides the United States, check that currency too, if you can. The card network will perform a double currency conversion in that case -- from foreign currency X to U.S. dollars, then from U.S. dollars to Singapore dollars. The double conversion has a little bit of cost to it, so you want to favor U.S. dollars a bit if the comparison is close. You only want to favor that other foreign currency if it's clearly better, by a percentage point or more.
You can check current currency rates using OandA.com and their currency converter.