to me NCP2 is the easy choice. As others have said NCP is less "complete" than AACW and would benefit much more from the improvements that likely would be inherited by VGN (diplomatic engine etc).
I would also add something about princing policy. Right now AGEOD games are by far the cheapest wargames on the market. For example AACW costs 20 € (or less) while its direct competitors, Forge of Freedom and Gary Grigsby's War Between The States are available only on the Matrix website and cost at least 36 € + VAT, and I know that prices for the american costumers are even higher. ROP is brand new and cost only 30 €!
Now, when you make a direct sequel (same time and place) the gamers value if the improvements and the added content is enough to warrant a purchase.
If these improvements are percieved to be too slight the gamer/costumer not only may decide to "pass" if he already ows he previous version, but he may be also tempted to purchase the first game instead of the latest if he doesn't own either one.
Matrix games limits this risk by keeping the price of older releases high, but as I said before the price of older AGEOD games has dropped a lot because the pricing policy is very different.
In other words, IMHO needs, more than others, to make sequels with relevant improvements or a big slice of the costumers won't buy it
