If we're talking about the first weeks of the eras then 25. She broke all-time records with the lead single and album and had huge sales. The success was unlike any other album.
1989 however was consistently exciting and the era didn't fizzle out so quickly. It started in August 2014 and she managed to keep it going all the way to Spring 2016 with 7 singles and with many awards won and records broken along the way.
25 is the bigger album but 1989 is the bigger era due to its longevity.