Amsterdam City Card purchasing better in US or once we arrive in Amsterdam?

Buy them there, preferably the MuseumKaart separate from a transit pass. Do reserve ahead for the Anne Frank House, if you're visiting there.

The Museumkaart costs you EUR 59,95 and gives free access to more museums in Amsterdam and the rest of the country than you can manage. It's already a good deal with 4 or 5 major museums, and saves you standing in the queue at the ticket window. This is the complete list.

The iAmsterdam Card is different from the Museumkaart and the transit passes. Which option is the best deal for you depends on how long you will be in Amsterdam and what you intend to see, as well as whether you will be going to museums in Dutch cities besides Amsterdam.

As for whether it's cheaper to buy the card online now or when you get there, just Google the price to find the equivalent (for instance, "40 USD in EUR" or "35 EUR in USD"). Don't forget to add any fees your credit card has for foreign purchases, or that your debit card has for foreign withdrawals.

Thanks to each of you. I forgot to post that we're arriving by cruise ship and will only be there for 10 hours. 8 to 6 pm. So I'm thinking the number of museums we can see is likely only 2-3. I've already seen the Anne Frank house so plan to skip it. Would like to see the Rijksmuseum, Van Googh and if possible the Hermitage exhibit. Plus eat lunch.

So with those limitations are any of the cards good for us other than we might be able to make reservations for the VanGogh and Rijksmuseums and skip the lines?

In that case buy your tickets online to avoid the queue at the ticket desk. Only the Van Gogh Museum has time slots. Have your tickets for early afternoon so you have ample time before and after. I'd say you should limit yourself to the Van Gogh and the Rijksmuseum. There is only so much you can take in on one day.
Have a leisurely walk from the cruise terminal to the Rijksmuseum. It' takes you one hour and is the best way to appreciate the city. There is not one street or one canal without its merits (the main drags excepted of course).

I agree with tonfromleiden - definitely buy your Van Gogh tickets ahead of time on their website. You will save hours standing in line.