Its true, in GTA there is no coop campaign, and there is barely anything else you can do, other tha run around get in a heli, or drive. Nothing si availible. 

In Saints Row 3, whether you are in Campaign with a friend or onine, the stuff to do never ends. You can do the entire campaign again, cuz its fun, or you can run around, buy property, kill random people with a tazer. Etc.

Saints Row > GTA.