Questions to ponder definetly. The Cobra is described as a good "all around" ship. It holds 18 cargo slots, but my understanding is that can be expanded to 60. So it seems like if you don't mind taking it to a ship yard you could reconfigure it without a cost penalty to be a hauler versus a fighter. Actually I'd have to see how this works in practise and the effect on your jump range with 60 tons of cargo and what you don't have equipped to achieve this configuration.
Since purchasing the Cobra, I've noticed it's not as maneuverable as the Sidewinder, but it has not been that much of an inconvienence. However, it you want a fastest turning ship and don't plan on trading any time soon, and don't mind less armor, well, the Viper might be a better choice, but I have not flown one.
One thing I've read that an advantage of the Viper is centerline weapons versus offset. I flew last night with symmetrical fixed multi-canons and was not happy with them. On the Cobra due to the converging stream, it seemed like many of my bullets were missing. This reminds me of my flight sim days wich machine guns mounted on the wings, you had to be at an idea distance for your target to be at the convergence distance. I did not notice this problem with the Sidewinder which also has non-centerline mounts but with a single gun on one side and a laser in the other, so until I can find some gimballed muti-cannons, I'm going to try an asymmetric configuration with both lasers on one side and both muti-cannons on the other and see how that works. Will report back.