How effective 5 cargo slots will be at trading really depends on what the average bounty cost will be, and what the average cost of a single item of commerce will be.
Also keep in mind that if you're trading, you'll have a much lower maintenance cost compared to someone regularly going into battle, so even if the cost of selling 5 items doesn't equal the money gained from killing in the same amount of time, it probably still evens out.
It would probably take an ace pilot who rarely dies to make an advantage there - but then an ace trader could probably find a good deal to make money out of just 5 slots as well.
Personally I always played Frontier as a trader, and always started with the fighter. Even if there is a low level cargo ship in ED I'd probably still do the same.