Looking at the Odyssey hanger and I can only hope the Polaris gets the same one.The volume of space a ship can scan is the product of the scan diameter area times the SCM speed of the ship. The ships are supposed to both have Large scan arrays, so given there are no other differences in scanning, the difference is all in the SCM speed.
I would not be surprised if the Odyyssey has an SCM near half that of the Carrack. So if it scans twice the space in three weeks and returns for more fuel, it really won't matter that the Oddysey does not need to return. In fact, spending time refining quantanium is time off task if you're really an explorer. Also, add drones and I think it is obvious the Carrack is going to be the much more efficient ship.
As for the Odyssey vs Carrack, I have a feeling that you are mostly right in that the Carrack beyond its wormhole finding ability is going to be more geared towards a military application exploring, fleet spotting where speed and movement are critical while the Odyssey is going to be more geared towards finding abandoned settlements/crash sites/Minable areas where long duration is more important. But this is not to say that both ships still have significant overlap and can both be used for each task.