+xHi, well, the good news is that it really wouldn't matter which you use. They're very similar materials and for use the size of a repair the difference in their performance and handling would be negligible. Quite simply, if you already have visible carbon/kevlar are the boat then you should choose the fabric that most closely matches the reinforcement that's been used on the area you're repairing.
Where the appearance is irrelevant then the 210g is a slightly heavier cloth so will add a bit more protection per layer. Being a 2x2 twill weave it also drapes a little more easily than the 188g and so is better around more compound curvatures. Conversely, the plain weave is a little tighter and less drap-able and so is easier to handle on flatter areas.
Thank you Matt, that explains the differences and how they can best be used on curves and flatter surfaces very well. Much appreciated