The main difference is usually that Mass products contain significant amounts of carbohydrates (mostly simple but also some complex). After realizing that as much as 40% of the weight of Mass shake mixes go to simple sugars like Dextrose, and another 25% to complex carbs, I moved on to normal protein shakes.

Serious Mass probably also includes things like Creatine and added Glutamine. These have been shown to increase mass building but see if Protein Optimum includes them. They are readily available supplements at most health stores which sell bodybuilding nutrition. I would definitely recommend sticking to the Protein and maybe added some sucrose to a post-workout shake yourself. Get the extra calories from things like meat and whole foods which will be better for you anyways. Finally, using Mass products is not recommended for longer than a month or two at a time due to the huge amounts of simple carbs consumed in a short period of time – its like drinking sugar with added starch and some protein.

edit – the reason I use Protein over Mass is simply a cost-benefit ratio. Serving sizes on Mass drinks usually recommend up to 4 large scoops both on resting days and workout days, costing you much more money in the long run. If money is not an issue, Mass drinks are premixed and have well-balanced carb ratios.
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