On my server, where the population is extremely low...
Convoys:
Pros:
You get lots of coins, and 30 coupons very quickly.
Cons:
You can get plundered once in a blue moon.

Plunders:
Pros:
You can get 50 - 100 coupons instead of 30.
Cons:
You spend 5+ hours trying to find SS convoys to plunder. If you spend that time working for minimum wage, you can buy farm more ingots.
Very likely to have negative coin income, as I will click red names for free coins ;P