My answer is definitely outside of what many BJJ practitioners (gi and nogi) believe. I say you should train NEITHER gi or nogi if you want to do mixed martial arts.
My response:
I say do actual MMA instead of nogi or gi. This is definitely not a prevailing thought right now... but MMA is three decades old and in these modern times we absolutely have a clear idea of what specifically works from BJJ, wrestling, kickboxing, etc. in the context of cage fighting. The sport of MMA absolutely has sport specific moves so train those. Only portions of the individual arts actually apply to MMA.
** It's a more efficient use of time to train sports specific MMA skills at the start to build your foundation.**
You can later delve into the gi or pure Muay Thai, wrestling, etc. to find something you can add to your foundation of sports specific skills.
The reality is majority of gyms don't operate like this. Most places offer a pot of classes focused on the individual styles and a sprinkle of MMA classes in the week. You're so lucky if there's a gym that offers actual MMA as something you can learn from day 1(as opposed to having to get in months of BJJ, wrestling, and striking individually -- and then needing to learn how to modify or ditch the moves that don't translate inside the cage.)
There's no reason why you shouldn't be learning how to deal with punches/elbows when grounded from the start of your training. And you should be learning how to use the cage from the start for takedowns/tkdn defense/getting up since the cage is a vital tool missing from the other disciplines. These are core MMA skills that are absent from both nogi or gi.
Danaher and Firas have stated numerous times you shouldn't be training in the gi if your focus is MMA. And then you have Craig Jones pointing out how much of modern BJJ isn't relevant in MMA. He makes these points again and again in his recent instructional offerings on BJJfanatics.
This debate seems like a neverending back and forth between BJJ traditionalists and fighting know-it-alls (admittedly like me.) Leave a comment with your take.