That is the perpetual question that always rears it's ugly head. Should I do train gi or no-gi (submission grappling)? Which is better? Aren't they the same? The argument is long and extensive on the internet. Both exhausting and boring and generally leading to personal attacks and so on.....so...
It's two different sports!!! Related with many, many similarities, but different. Different rules, different uniform, different games. But still very similar. Whether one is better than the other is really a matter of personal choice and goals.
So...the question is not always so much which is better but how are they similar and different and then how do I translate from one to the other if I choose to do so. Here are some of the differences that immediately come to mind.
TECHNIQUES: There are many techniques that are based purely on you or your opponent wearing a gi. (i.e. collar chokes, sweeps utilizing the belt to hold on to, etc.) And there are many techniques that work only or better without the gi (i.e. the collar gets in the way and makes it difficult to sink a choke in). Some techniques have equivalents or need slight adjustments to make them work for both games. Others are gold in one game and completely useless in the other. Learning which techniques work for which game comes with time and practice.
HANDLES: The gi gives you handles. You can grab the pants, hold the lapels and sleeves, grab handfuls of gi to help gain control, initiate sweeps, etc. The handles can slow the game down. This can be beneficial for beginners who need to learn at a slower pace and learn the basics...positions, basic techniques...more handles = more controls. Without the gi, there are no handles to hold on to and you have to create other ways to hold on and control. You can use your hands like hooks on the elbows, shoulders, legs, ankles, etc.
ATHLETICISM: Obviously good for both since this IS a sport. But pure athleticism can compensate for lack of technique in no-gi moreso than in gi. Athleticism is also beneficial to the younger, stronger and quicker guys. Of course, the flip side to that is that more injuries can happen as a result. For example, sometimes just using strength to bust your leg out of a certain leg control can result in a knee strain or worse, a torn ACL. Of course, athleticism can be beneficial in both games and injuries are also going to happen in both games.
RULES: Generally in the higher levels of no-gi, heel hooks are allowed. Sometimes bicep locks/slicers are legal as well. No-gi usually doesn't have advantages in scoring or give points for knee on stomach. Of course, every tournament is different so read the rules carefully. Pay attention for different rules for different ability levels.
PROXIMITY/CONTACT: Sometimes you can keep a little more distance with the gi. With no gi, you have to use your body to control instead of relying on the handles to pull in or keep distance so your bodies will have (even) more contact.
TAKEDOWNS: Gi based jiu-jitsu takedowns are generally based more in judo since they share the similarities of the control handles. No-gi is generally more based in wrestling where there is pummeling and tieups and no gi to hold on to.
SWEAT (sometimes called "greasiness"): There's a lot more sweat and bodies get slippery or greasy after rolling for any length of time without the gi. The gi can help absorb a lot of the sweat and provide friction when contact is made. Without the gi, sweat is just slippery and uh...drips. Rashguards can only absorbs so much. Gi's are more like Bounty!
So, depending on what you want to do and what you enjoy, choose which game best suits your needs or just do both!