Charles V has far less ability to travel to London and back occasionally, or Mary to Vienna. The match may make good political sense in the short term, but a paper marriage almost certain to produce no heir is in no one's interests. Phillip is indeed still the most probable match.
If there's only one child, he or she becomes King or Queen of Spain and the English crown would pass to Mary Queen of Scots according to Phillip and Mary's OTL marriage contract; Englishmen would probably demand the same clause TTL. No personal union would be contemplated.
Edward Courtenay, Earl of Devon is probably in second place if the anti-Spanish faction or Mary's whims sink the Spanish marriage.