Oh yes, the age as old as time itself...
Or, at the very least, close to it: should I train Jiu-Jitsu in the gi or no-gi?

Of course, the answer first depends on what you're trying to accomplish.

If you're a hobbyist that wants to train for no other reason than to be fit, have fun, and keep improving at your own pace, then by all means do whatever you fancy most.
On the flip side, if you're trying to become extremely good and win championships, well then again you have to decide if the gi or no-gi suits your goals better.

But here's my personal opinion, that you might or might not like.
Train both.

Sure, there are arguments to be made against the gi, which say that it's unrealistic, that it slows the game down, and so on.
And with the present trend of rising no-gi Jiu-Jitsu popularity, there is a danger of gi BJJ becoming ostracized completely.

But if you want to become a complete martial artist and if your ultimate goal is to fulfill your potential as much as you can...
Then you should train both. Because, unrealistic or not, the gi will help you develop your skills so much better.

How? There are plenty of answers for that one, but perhaps the most important one is resistance.

Because, when someone has those gi grips on you and you feel stuck with them, that forces you to use your technique more. It forces you to think, rather than act on impulse.
You cannot just disengage and stand up in the gi. You have to be there and mind your way through it.

By presenting more options, both offensively and defensively, the gi will force you to develop a more nuanced and technical game...
That you can then combine with the explosiveness and fast-paced nature in no-gi.

The gi will complement your no-gi training and competition.
So if you're serious about improving, go ahead and train with it. At least once per week.