Neither, as yet, have ISO codes assigned.
For some uses, considering it part of the Netherlands (NL) will serve, as they are constituent countries. Note that "constituent country" isn't a rock-solid distinction as such matters go, and it's quite common to e.g. consider the United Kingdom of Great Britain and Northern Ireland as a single unit in a context where one might also consider Denmark, Greenland and the Faroe Islands as three separate units.
For others, encoding as "Former Netherlands Antilles" will serve.
For other uses, this would not be acceptable. My recommendation would be to use codes from the user-assigned area of ISO 3166 (AA, QM to QZ, XA to XZ, and ZZ, though you may wish to avoid those in common usage in public contexts), and reassign to the code assigned later, if that happens.
Note also, that it may be worth keeping Netherlands Antilles as an option in some cases, as Netherlands Antilles is obviously the place where some things happened in the past which may remain worth noting for some time to come (one reason why many complained that CS was re-assigned from Czechoslovakia to Serbia and Montenegro too soon, is that valid and useful data about Czechoslovakia was still in active use at the time).