Either way is fine. If you want sand, it should be ok although you may have to stir it frequently to prevent it from getting compact. Cories tend to forage the bottom a lot so they lessen your burden to disturb the sand.
As for gravel, make sure you buy gravel that do not have sharp edges as this can damage the cories' barbels.
Gravel is safer for your filter, and your drain when doing water changes. Be careful that the sand doesn't get into the impeller assembly, it will fry your filter quick, and going down the drain during water changes... will clog the plumbing, and it doesn't take much.
The smaller grade of gravel is fine for the corys and safer for the filter because it's heavier, doesn't get pulled in through the intake to damage the filter parts. The gravel is also easier if your're working with live plants, as the sand is too dense for many plants to grow in properly. Sand would greatly limit your selection of plants.
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