I like sand, with the bigger gravel it was easier for the fish to uproot everything. The sand seams easier for the the roots to stretch out in, and not one uprooted yet and it's been over a month.
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Depends what type of plants you want to grow and what your after?
If its plants with high demands, then i would say neither, and instead go with a decent type of planting substrate, since it will save you some problems in the long run...