Plastisol inks are very thick inks that holds a lot of pigment in it. This allows for extremely bright and bold prints. Plastisol ink also has a large variety of specialty inks or additives like shimmer, reflective, glitters, puff, glow in the dark etc.
On dark t shirts, a white under layer of ink is printed first then brighter colors are printed on top Printing under base layers is similar to using a white primer on a wall before painting over it with your color of choice. This allows colors to show bright even though they are printed on a dark background.
Water based discharge inks work by bleaching or removing the dye of a t shirt to it's original cotton color and replacing it with the pigments in the water based inks. This can affect the color in a couple ways.
Firstly, some colored t shirts bleach better than others. T shirt colors like royal blue, or kelly green, may not discharge as well as an indigo blue, or forest green and will result in different than intended print colors.
Secondly, discharge inks set into the fabric and dyes the thread fiber so naturally, the colors will be a bit muted. This is different in that plastisol inks sits on top of the threads of a shirt, as it's only layer of pigments of color which allow for brighter prints.