Monitor Audio G4 Gold 100 or Proac Studio125?

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.

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

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 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.

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