I would think Spain. It was always Philip II's interest to keep England onside, as he wsa fighting the Dutch rebels. However Elizabeth proved first vague and then downright hostile, hence part of the reason for the Armada. Philip never really wanted England as a fief; he had enough problems of his own, like massive debt, the Turks, the Dutch etc (to name a few). Have silly Mary Stuart on the throne and things with the Spanish get cosier...
The Stuart line is indeed the one there that stands to inherit.
Could be iffy though, their position in both kingdoms would be unstable.