TL;DR :

MQA :
* is lossy
* is proprietary
* produces smaller files than FLAC
* can only be played on compatible equipment, having the $$$ codec

FLAC :
* is lossless
* is free

I was initially reading https://danielmiessler.com/blog/the-truth-about-high-resolution-audio-cd-quality-and-mqa/ and wondered what MQA was ;-)