I believe it's a fairly long flight from the US so you may like to rest up for a night or two in BKK before heading off. Also, as you say, you really need a crystal ball to accurately predict weather, often you don't know what's happening until you're on the ground. So why not land, stay a couple of nights in BKK and check out the weather then, and make your choice between Phuket or Samui based on the current reports.
Samui is a nicer tropical island experience than Phuket so I suggest you place it on top of your list. Samui also has numerous spas, the island has shaped itself into a spa mecca in recent years. One I know well is called Tamarind Spring and it's at North Lamai. A fantastic place, you'll love it.
PS: I've responded to your message.