When comparing Sony a6400 vs Olympus OM-D E-M10 Mark II, the Slant community recommends Sony a6400 for most people. In the question“What are the best mirrorless cameras under $1,000?” Sony a6400 is ranked 8th while Olympus OM-D E-M10 Mark II is ranked 18th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Good autofocus
Pro Good electronic viewfinder
Pro Good 4K video
Pro Good battery life
Pro Great focusing system
The focusing system is quick, it rarely if ever resorts to focus hunting and it's consistently accurate.
Pro In-body stabilization saves money on lenses
Since stabilization can be applied to any lense, you can save money by not having to spend it on stabilized lenses which are generally more expensive than non-stabilized lenses.
Pro MFT lens mount allows for a wide variety of lenses to choose from
Micro Four Thirds system is a standard used by Panasonic, Olympus, Sigma and many others so you're not locked into a having to buy lenses from one manufacturer.
Cons
Con Poor 1080p video
Con Can overheat in 4K mode
Can overheat when filming in 4K mode, see: https://youtu.be/ytzWlrEgGPE
Con Rolling shutter artifacts
Con Menu system could be easier to use
The camera menu has 26 screen pages and dozens of subpages. Navigating through them to find what you need, can take a while.