i'd do both bro, but the weightgainer will get the gains quicker. try mega mass 4000 if u have a hard time with putting on size! it's good stuff! 
Actually, it's a fairly good product. I used it myself for about 3 years, when I was in college, interchanging it with the 12-lb bags of Mega Mass 2000 (depending on which was cheaper). Though both are quite carb-heavy, Giant Mega Mass is a bit more protein-dense then Super Mega Mass is.
The one knock is that the protein quantity varies based on flavor. I believe Vanilla has more than Chocolate and Pineapple has more than either one of them (but, the only place I've seen that flavor is in Spain).