As an example:
64GB SanDisk Extreme - Read Speed: 160MB/s, Write Speed: 60MB/s, Video Speed: C10, V30, U3, A2, Form Factor: microSDXC
64GB SanDisk Extreme Plus - Read Spead: 170MB/s, Write Speed: 90MB/s, Video Speed: C10, U3, V30, Form Factor: microSDXC
The Extreme Plus is capable of slightly faster Read Speeds, and a third faster Write Speeds, than the Extreme.