There is no sense in which the regenerated part was present preformed in the amputation stump. Yet in a sense we now know preformation wins as well - because the instructions for making a frog, rather than the frog itself, lie in the DNA of the frog's egg. Yes of course they are, because in most organisms development can proceed, at least for a time, without the zygotic nucleus. We know that zygotes contain copies of all the genes in the genome, and pass these on to their descendants. We also know that not all genes are active in all cells.
Lets now look at genetic and non genetic factors.
Although they are likely to be complex and varied, and although we can't quite specify what they are at this stage, if there is any truth in epigenesis we should be able to see development as a hierarchical decision making process.