I strongly believe that training with the gi is better for improving grappling skills, even for MMA or no-gi grappling.
It is often argued that training with the gi tightens up your defence, with greater control over your body it will be necessary to use good technique to escape a sub.
It's hard to argue against this logic but it should be pointed out that by the same reasoning training no-gi will also tighten up your subs, a slippery opponent is harder to control after all so I don't think this argument is particularly strong overall.
Instead my argument focuses on the fact that there are so many more techniques and positions available with the gi. In my opinion the knowledge and experience of all these extra techniques will have a positive effect on grappling ability in the long term.
I understand that with different grips and strategies there are a lot of these "extra" techniques that many would consider irrelevant to training no-gi but I would argue that this is just a matter of diminishing returns. If we assume that there are 3 times as many techniques available with a gi then maybe the long term advantage would only be 2 or 4 %, it's almost impossible to measure but this kind of margin is certainly enough to make the difference between winning and losing.
When we look at the evidence it is clear that training with the gi produces the best grapplers in the world, Roger, Marcello and so on are all exponents of training with the gi.
If you want to make short term gains for training MMA (when you have no grappling experience) then I would recommend drilling techniques without a gi.
If you want to make short term gains for MMA or no-gi I would recommend training with a gi and looking for all of your no-gi subs (it's amazing how easy it is to sink a brabo or rnc for someone coming from gi to no-gi)
If you are interested in competing at the highest level in MMA or sub grappling then I suggest you buy a gi and start your journey to black belt.
All my humble opinion, feel free to voice your own.