Stumbled across this thread and I'm in the same situation, land at AMS in the morning and need to head directly to Antwerp.
I see on the ns.nl website there are trains from Schipol to Antwerpen pretty regularly. Do these trains get filled up or would I be fine buying tickets at Schipol once I land?
Also, I have a US based credit card so it's chip+sign and not chip+pin. Will that be an issue buying tickets for trains/public transport in Holland?