Both spoil in 60 days and are otherwise identical in stats. Tallow takes 3 chunks of fat and gives 2 charges, lard takes 2 chunks of fat and gives 2 charges PLUS some cracklins, which are a decently tasty, though unhealthy, food item. Both take 1 water or clean water, plus fire or 7 charges of a heating item, plus cutting and basic cooking. Any recipe that can use either uses the same amount of either.
Lard appears to be flat-out better in terms of efficiency BUT there are a handful of things that can be made with tallow and not lard, and there might even be a couple that can be made with lard but not tallow. Personally, I’d focus on lard but make a few hunks of tallow just in case.