I suggest neither... Get a proper dedicated media player.. HTPCs, Android/Android TV, RPI.. You'll get much better performance especially where Kodi, high bitrate 4K, X265, etc files are concerned.. However, if those are the only 2 options your are interested in.. Then go for the Fire TV Stick.

As before.. completely rule out your shortlisted devices and grab from below..

1. Nvidia Shield
2. Minix Neo U9-H
3. Mecool KM9
4. Vero 4K+ (OSMC)
5. WeTek Hub and Play2
6. RPI3 (This is fairly powerful but it's more of a DIY route.. Based off your questions, I'm going to assume that you aren't ready for such tinkering yet.

These be my recommendations and in order of preference... For smooth 4K HDR + HD audio (passthrough/otherwise), I can vouch for the top 2 on the list.. You'll also require their Gigabit LAN and auto Frame Rate Matching for silky smooth 24p video playback.

r u talking about online 4k hdr streams with kodi addons via realdebrid?
really, even if the devices are capable, are such 4k hdr streams easily available and reliable with kodi addons NOW?

even with vpn, realdebrid.

if so, i am surprised... since with the european laws now set to change to punish/fine those who offer pirate, i wud think few dare to continue doing it.

more so for high quality streams since the prosecutors will probably target these in particular.

i may be wrong but wud like to know the current state or affairs.

I changed my mind.. I'd go with neither... If you are looking to purchase. The MXQ has a newer OS version and SoC, but it's 1GB RAM & 8GB ROM in this day and age is...silly.. The MeLe M9 seems to be a much older device..at least since 2013 and it wasn't a great device to begin with.. Pass. So I can't in all honesty recommend one over the other.
That can happen too..