Monitor Audio G4 Gold 100 or Proac Studio125?

It’s personal, subjective, and very room dependent.  Try them both, and keep the ones you love the sound of in that setup.

I agree with @mofojo keep the ones you like.  I’ve not heard either, but if it were me, I would spend plenty of time listening to both playing all the various music I like, particularly the genre I listen to most often, and see which one suited my musical tastes the best.

Apples and oranges

Monitor Audio is 5 times bigger than ProAc (economies of scale). On the other hand, ProAc  https://speakerchoices.com/?brand=proac products are higher end. 
Resale value: Monitor Audio

Quality: ProAc

Sound: whatever you hear....

I’d keep the one you like better if I was you… 

I looked up ProAcs you mentioned and based on my experience on completely different models from ProAc and Monitor Audio, I would recommend the ProAc. I find them to be more “exciting” than the Monitor audio. But again this is a personal preference. Another person might have the complete opposite preference. Good luck.