Albert Barnes Commentary Mark 8:15

"And he charged them, saying, Take heed, beware of the leaven of the Pharisees and the leaven of Herod." — Mark 8:15 (ASV)

Beware of the leaven of the Pharisees. See Matthew 16:6. He also warned about the Herodians—meaning Herod and his followers. Matthew, instead of "Herod," has "the Sadducees." It is probable that He cautioned them against all these groups.

The Pharisees sought His life and were exceedingly corrupt in their doctrine and practice; the Sadducees denied some of the essential doctrines of religion; and the Herodians, it is supposed, maintained the opinion that it was lawful for the Jews to acknowledge a foreign prince and to join equally with the Pharisees and Sadducees in opposing the claims of Jesus.

Matthew recorded His caution to avoid the Pharisees and Sadducees, and Mark also added the caution to beware of the Herodians, which Matthew had omitted. Thus, the evangelists are saying the same thing.