You're looking for a "general purpose" ship to run some cargo and explore the 'verse. The Corsair is cheapest, and has the most guns, but the least cargo space. The Mercury is probably the fastest, not much more than the Corvair in price, and has all those lovely computers. The Aquila cost the most but is still in the same price category (unlike the 600i and Carrack) and arguably the most adept at exploration of the three. Why choose one over another?