I have read different responsa to the question of scotch that is aged either only in wine/sherry casks or firstly in a wood cask followed by a second aging period in a wine cask. Given the discussions in the Gemara and shulchan aruch (which seem to permit at least the latter category), what are the differences of opinion based?

The Shu"t Minchat Yitzchak (V 2; C 28) writes that he cannot pasken in either fashion. He brings a list of reasons to allow of forbid drinking whisky. If he thought that it was really forbidden he would have concluded the disccssion with a final Psak. Therefore one it is permitted to drink all whiskey. A Yereh Shamayim though, might prefer not to. Rabbi Zalman Baruch Melamed