IMHO, Nethergate's a better game -- more interesting, more innovative, and a story better told than A4. If you're looking for raw game-hours before boredom ensues, A4 is a better bet ... unless the whole game bored you. But you're easily likely to get 100+ hours of gaming out of Nethergate between the two sides, even playing each through just once (and no, there's probably not much replayability beyond that, unless you're a shameless min/maxer).
Better yet of course would be to buy both, but I'm operating under the assumption that that's not a financially reasonable option for you.
Just my $0.02 -- enjoy! And I'll leave it to non-lurking regulars to extend the traditional Spiderweb salutations with regards to where your sanity is best left.