I think Porte is wise to stay with Sky.
other than the probable big wage, which makes a big difference at the end of the day, hes in a very successful team and he could no doubt see his improvement before we did via results.
gets to try one week races as leader, wins and podiums. Logical next step is GTs, if not the tour. Strong team (riders and support) around him, so why jump to a smaller team and possibly get hung out to dry if he doesnt immediately succeed? English language and culture fit easily too.
Sky probably see him as a long term fit within their structure and are very happy to have such a strong number two who is loyal. That cant be underestimated.
He probably also enjoys riding on a winning team, thats always good for personal morale.