TLDR - Hidizs MP145 

Akros made that comparison recently

Heyday is way better IMO, much better clarity, more neutral sounding. Very disappointed with the MP145, comes across very dark and low res, even using the brightest nozzles. Heyday is a great balance of neutral and not too intense tuning but retaining that planar clarity. MP145 goes too far muting the upper mids and treble and it loses that planar magic. The fit is also appalling, Heyday is a lot better.