Difference between the restaurant and the market
I don't have Tropico 3, so I'm basing this off Tropico 4 (which I've heard is quite similar). I believe the main difference is that a restaurant can have an entry fee (which means you'll make some money back), but it also has a higher initial cost and higher upkeep. If you use the entry fee to try to offset the higher costs, then only Tropicans that have enough money available will be able to utilize it. So if you have a large number of low income Tropicans that don't live near food sources, you may still want a market to help them get their food.