Pennsylvania a Keystone State? Maybe but Pennsylvania is not a state, officially speaking.
Pennsylvania is a commonwealth.
In between the North and the South? Yes, but New Jersey has a more dynamic reach.
New Jersey is at latitude with the Mother of all Southern States - Virginia, and the Father of all Northern States - Massachusetts. Pennsylvania is only on par with Massachusetts. New Jersey is at equal latitude with all of Nantucket and Martha's Vineyard, while the southern point of NJ is on par with Alexandria, Virginia. Thus New Jersey is the only original colony state (and not a commonwealth) at both extremes, unlike Pennsylvania.
Rather Keystone Commonwealth fits better for Pennsylvania. Thus New Jersey is the true Keystone State of the original 13 colonies in between the North and the South.