Definitely the Isolate, since it is the purest form of protein. There are other products that claim to promote mass increase, but they usually have a shitload of sugars and other additives for this purpose, which is not cool. Meanwhile, the isolate has no carbs, no fat, and no lactose. If you manage to find a product with 100% isolate and no additives, it would be awesome. Personally, I love the Isolation by n2bm, especially the colombian espresso flavor - it has no artificial sweetners, and the taste is great. You should definitely give it a try.