You overlooked the calories: 180 in Slimfast, 400 in Solyent. And that's just the premade drink. One pouch of the powder is a flat 2000, something I don't think Slimfast even has an answer for. Also, pay attention to the ingredient list: Milk, water, sugar, in that order.
Slimfast is a diet drink as the name implies, while Soylent is a meal replacement. If you wanted to get a reasonable (non-starvation) amount of calories from Slimfast to live on it and it alone, Soylent then wins on balance and cost.