Well assuming it is one of the vanilla saplings. Oak with Steve's Cart would be the best, since it would be based on the amount of leaves and drop rate. Of the 4 vanilla trees: birch, oak, and spruce have a 1/20 chance of dropping a sapling, jungle is 1/40. Using forestry the machine automatically limits the heights of the trees you can grow, however steve's carts doesn't limit it and can have the fertilizer module if you have access to a skeleton farm for bonemeal allowing for faster growth without the forestry forester. Because Steve's carts doesn't limit the height of trees you can get the very large oak trees which will likely give a larger number of saplings than birch due to it's limited size.