Yes train gi. There are a ton of high level black belts who say to training in the gi just makes your overall game better. There are only a few black belts who promote only training no gi, we all know who. I have rarely trained no gi like 1 time/month maybe, but I feel I can drop in anytime and hold my own there, but if a no gi guy comes to gi he is like a fish out of water.
I cant speak for all schools, but in my personal experience. The group of people who train no gi use less technique and more physical strength, and just generally the overall quality of the students is higher in gi. It also slows down the game and makes you think more. Going fast doesnt make you better, it just makes it more likely for injury.
Also in no gi it is really irritating for gi players, because people dont do basic moves sometimes. I have run into several people who dont break guard correctly in no gi, or who skip steps in their moves.
I feel in no gi people tend to get while and flail a bit more and i always end up taking a knee to the face or a hand to the head. That NEVER happens in no gi, though I do get a lot of gi burns. Injury/time ratio is much higher in no gi for me. And fastest way to slow down progress is injury.