I went with Spieth. Serena is the most dominant athlete in any sport (men's or women's) ever in my opinion. It wouldn't shock me at all if she did it, because she's so much better, even at her age. It would still be an incredible feat either way though.
Whereas everyone else in tennis is a lesser opponent for Serena, there are a lot of golfers who you could argue are Spieth's equals. For him to win four majors against his equals, it would be unbelievable because he would need to be nearly flawless those four weekends.