The maximum bitrate for the a6400 is 100mbps, or 12MB/s. Sandisk Ultra cards are only rated for 10MB/s continuous write speed, so wouldn't be suitable for your camera. Sandisk Extreme carry a V30 rating which means they are certified to support continous write speeds up to 30MB/s, over twice the maximum bitrate for your camera.
The Differences Between SanDisk Ultra And ExtremeIf you’re looking for a new memory card, the SanDisk Ultra and SanDisk Extreme cards are a great option, but
The world’s fastest 1.5TB1 microSD card is perfect for storage expansion, not only for Android devices and tablets, but for Windows laptops and Chromebooks as well. It provides a future-proof solution for storing massive media collections--high-res photos, videos, and music, plus podcasts, documents, games, and shows from streaming sites like Netflix.

SanDisk, the SanDisk logo, Memory Zone, MobileMate, SanDisk Extreme and SanDisk Ultra are registered marks or marks of Western Digital Corporation or its affiliates in the US and/or other countries. The microSD mark and logo are marks of SD-3C, LLC. Android, Chromebook and Google Play are marks of Google LLC.

My current card is a SanDisk 4GB micro SD that's class 4, takes a while to back it up on my computer for only 2GB of memory. Size isn't a real issue to me cause I don't buy a lot of games digitally that take up a lot of memory. I've ordered a SanDisk Extreme PLUS 16GB micro SD UHS-1/class 10. . 114 308 167 410 239 9 187 204

sandisk ultra vs extreme microsd