I already have this pan. 2.5mm is the way to go. This is a great pan. This is the original cast iron handle. There are issues trying to buy the whole set with this handle, as I understand that they do not make this handle any more. Mine has this original handle and we love it.
JCJCJCEven if you don't get the old style Cast iron, the cast stainless with electroplated iron is a nice substitute and you'll still be getting an amazing pan.