I don't believe that one or the other is easier to get to from North America. Amanpulo is accessed via Manila. Amanwana is accessed from Singapore via Bali. You will have a layover whichever way you go. But there are good airline and hotel options for both so neither is a real hardship. Aman will make air/boat arrangements for your final leg of travel for either one.

Service is very good at both.

Amanpulo:
Very good snorkeling and diving
Private, small island
Lovely casita accomodations
Spectacular, sugar-sand beach
Beautiful views all sides of island
Not much hiking, too small
Wildlife mainly birds, turtles, geckos
Full menu for dining
Generally more luxurious

Amanwana:
'Planet Earth'-level snorkeling and diving
Private location on a larger island
Tent accomodations (more rustic than Aman-i-Khas)
Crushed coral/shell beach
Nice views one direction
Lots of varied wildlife, too much to list
Lots of hiking options, bat cave, etc.
Generally more rustic
Simple chalkboard menu, but happy to make other selections as able
Easily combines with other Amans in Indonesia/Bali