Aww, the age old question (in BJJ)... "To Gi, or not to Gi?"
The most common arguments pro, and then con gi (with counter arguments following immediately after):
Pro Gi training:
*Training with the gi is a far more technical game. There are far more options for attacks (gi chokes, and grips etc.) so you are forced to develop high levels of defense, because it is just plain harder to get away once caught.
-While we concede that it is true that the grips and friction of wearing a gi can make it harder to escape submission attempts, it is also true that training in the gi can create bad gripping habits that are useless when your opponent has light clothing or if you decide to fight MMA. Gi grappling relies too much on these grips so a lot of your attack set ups (which rely on grips) will no longer be valid. Also, if you train in the gi you are use to a much higher ability to control your opponent with your grips. This control with out the gi is much harder to maintain without gi grips, and especially against an athletic opponent.
Con Gi
*People don't wear gi's on the street! So, by training no gi you don't learn to rely on grips that you may not have if the opponent is wearing light clothing that will tear, or no shirt at all. Since you don't rely on clothing grips, everything you learn will be applicable and so you cut out the unnecessary.
-While we concede that no-gi techniques are still applicable when an opponent is wearing clothing, and not always vice-versa, it is important to acknowledge that the dynamics of rolling in clothing is different than rolling no-gi. To only train no-gi you will be unable to develop proper defense because the lack of friction caused by the clothing make many attacks easier to avoid.
Okay, my take:
To be a complete grappler you really should train both gi and no-gi.
Gi training (most of the time) is more technical, because of the reasons stated above, but also because much of the explosive energy and strength of the natural athlete can be negated through grips. The game becomes a bit slower moving so you can really begin to feel what proper body positioning should be like when you do certain types of techniques (this kinesthetic awareness that I find you get more often with the gi translates directly to the no-gi game).
You should train no-gi to understand the importance of explosiveness (in the proper places), and for the overall quicker pace of no-gi rolling. You have to learn to apply techniques on the fly, and learn to recover and attack again after failed attacks. These fluid quick style attacks can help improve the same kinds of attacks when training with the gi, and the overall more athletic nature of no-gi training will help you understand better the advantage to that kind of fitness.