This is a question many people use to ask my advice and in my opinion, Gi and No-Gi are 2 completely different universes.
Why do I say that? Because training with a Gi your possibilities of variations and reactions are endless and you will prepare your mind to assimilate the techniques with a better understanding. Also, the transition from Gi to No-Gi is very easy and smooth while the opposite is very complicated and many people get totally lost.
It’s very rare people who train No-Gi feel comfortable to put a Gi on to spar, and I’ve never heard about a person that only train No-Gi to win a gi tournament in high level divisions. Now, when you talk about guys that just train in a Gi competing in the most prestigious No-Gi Tournaments (Abu Dhabi) you will find them winning every single weight category.
I’ve been involved with Jiu-Jitsu since 1970 and I’ve experienced myself training without the Gi with the best of the best Luta Livre (ground fight system practiced in Brasil without the Gi), and I can tell you that even with only 3 years of experience of Jiu-Jitsu at the time, I’ve had epic battles with the top Luta Livre guys with over 20 years of experience including the founders, and I use to invite them for a friendly Gi training, my invitations they were never accepted. I think they just lacked the confidence to enter the Gi World.
My advice is to start your training in a Gi and focus on the basics always.