What's the biggest difference between the Odyssey and the Carrack?

The biggest difference is the Carrack's specialization in various areas, such as jump point scanning/mapping, onboard drones and repair facilities, and its three modular cargo bays that can be outfitted in the future for various purposes beyond cargo. The Odyssey, however, favors a more generalized approach, with the flexibility to do a lot of separate roles depending on how you equip it with onboard ships, ground vehicles, and supplies.