To put things in perspective,
If Brazilian Jiu jitsu was a game, no-gi would be checker, and gi would be chess.
Now, do you want to learn how to play checkers, or chess?
Personally, I started training BJJ in no-gi, and I did so for about 1 year and a half.
I started gi about 2 months ago and I would never see myself getting back to no-gi. Now, about your situation, I understand that no-gi is probably more concrete as you are into MMA, but keep in mind that all of the best no-gi champions are gi champs, and mostly train with a gi on.
Training with a gi will not slow down your MMA training, it'll just make your jiu jitsu game way more technique (and less based on strenght) than if you were to train in no-gi.
My 2 cent.
When we go to the ground, you are in my world. The ground is the ocean, I am the shark, and most people don't even know how to swim. - Rigan Machado