Apples and Oranges really

First, Mest mkii sound changes a lot between tips and insertion depth, it goes from sparkly, to mid forward, to warmish with excellent bass impact. Oriolus reborn not so much. I’ll go with pros and cons (to my ears and with the tips/sources I’m using)
Mest mkii:
+ Holographic soundstage with sounds floating all around your head
+Musical and technical at the same time
+Emotional vocals
+Bass quality and impact
- A LOT of tips rolling required. They fit more deep than Oriolus so you may feel them more than oriolus. Funny the best tips I found for them were the silicon Oriolus tips
- Vocals in some songs can be intimate. That may be good or bad depending on your personal taste
Oriolus Reborn:
+ They are supercomfortable as the fit is shallow. Especially with spiral dots
+ Musical. VERY musical tuning with warmish bass and lush mids
+ 3d soundstage
+they make you sit and want to listen more and more. They are very euphonic.
- Treble is not as extended as mest mkii
- shallow fit may not be for everybody. They are also bigger than mest mkii but not excessively so
- while soundstage is large and 3d, the mest mkii soundstage is still more holographic
- mest mkii is a more technical iem, reborn is more about musicality
- they have a more present subbass than mest mkii which make them slightly warmer and lush. This can be good or bad depending on your taste
I would add that imo the mkii/reborn trumps for musicality a lot of superhyped iems released in the past years. Unfortunately it seems to me that Oriolus is not very interested in marketing here or other forums so the mkii/reborn didn’t gain as much interested or following like some other brands iems (i.e Campfire or Empire ears).