For us we decided to go with Helium mostly because of lenses. We want to use Canon cn20x50 zoom which has a whopping range of 50-1500mm. However it only covers 5.5-6k on Monstro and on Gemini about 4.5k. Helium it covers about 7.5k (sometimes 8k)

4.5k just felt too low compared to Helium 7.5k when you want a 4k output. After using the Helium it really impresses me how sharp the you can make. You get an extremely sharp 4k output which I could never get with the MX at 5.1k. At 4.5k it would be worse again.

Monstro is about 1.3 stop better when it comes to noise and seems to have a bit better colors. For us it is just not worth it beause how much it limits our lens choice.
Pulling focus at 8k is frustrating though when we only have a 1080p output. UHD output would make a big difference.
/Andreas