Both of the above are commonly taken supplements, both will help you to gain limited amounts of muscle. But, supplements aren't nearly as important as eating enough protein and quality calories.
Buy a weightgainer if you are serious about wanting to put on muscle, it will contain a lot of protein, complex carbs, and fiber (many also contain creatine). You can't build much new muscle without eating more, so do this before you look at supplementing much.
Weightgainers will put some fat on you, too. Usually I can gain about 20 lbs in a month on a gain, and 1/3 of that is fat. So you should get to a point where you're OK putting on some weight around your middle.
If you cycle on them and do low cardio low reps high weight high rest, then cycle off and onto whey protein high cardio moderate reps moderate weight moderate rest. You can put on and keep muscle while managing your body fat.
hope that helps