The unlimited Metrocard can be bought at any Metrocard vending machine (both at AIRTRAIN termini and at regular subway stations). It cannot be used for the AIRTRAIN. The cash-value Metrocard can be used on both the subways and the AIRTRAIN (and city busses). The unlimited AIRTRAIN card (which I gather is on Metrocard stock) would be usable only on the AIRTRAIN. This is almost certainly designed for JFK employees and very frequent users of JFK airport. This would not work on city subways nor city busses.

If you want only one card, get the cash value card, but crunch the numbers first.

Even at $2 on the cash value card (actually 20% less than 2$ if you buy cash value cards in increments of $10 or greater), the subway is a great value.