RedvenThey're pretty similar as they're both a take on neutral but the MD+ has some minor lift in the sub-bass and upper region region and the Orion is overall more flat. Also, single BA vs 3-BA in the MD+. Orion is also more expensive by $50. Guess it really boils down to personal preference.
RedvenImo, they're both tuned in reference to the ER4. (CA doesn't explicitly state this but Orion's sound signature is almost identical to the ER4; especially ER4XR). Neither offer the sublime clarity or isolation of the ER4 but both make some changes that make them sound less analytical while remaining close to neutral.
The Orion sounds slightly warmer and has a bigger sound stage while maintaining a flat signature. Treble and bass have good clarity with extension that is adequate but limited by the single BA configuration, similar to the ER4.
MD plus offers a sound stage smaller than the Orion but bigger than the ER4 while also increasing extension; particularly in the bass region. The vented 2BA for lows gives it a nice, airy punch more typically found in dynamic or hybrid IEMs. This however, creates more recessed mids compared to the ER4 or Orion.
Ultimately, it depends on your own listening preferences. If you ever felt like the Orion could do with a touch more bass and sparkle, I think the MD plus is a great choice.
Also a side note, while the Orion's are reasonably comfortable, The MD Plus are simply one of the most comfortable UIEMs I've ever used and their design makes them compatible with a wider range of ear sizes. I've actually started using these over closed-back headphones and CIEMs for long plane rides.
Ll_FFor this price range, I personally like Audeze isine 10 or Orveti new Primacy.
If you're willing to invest a bit more, Isine 20 or UM Martian.
If price is not a concern, Oriolus V2. There's also the Fender FXA11, it might be kinda hard to find outside of Asia though.