I'm thinking on which Princess (Saxonian or Danish one) is better. The Saxonian match is the most butterfly-safe one (though still with butteflies in form of York-Cumberland duo staying in command during Second Dutch War). The Danish one quickly creates the ripple of butterflies through the Europe.Erdmurthe Sophie died in 1670, so even if she does marry Gloucester and dies at the same date it opens him up to marry again.
But those two are the highest ranking Protestant princesses available as of 1660.