Here's the deal on the backpack areas for the 200 and 300. The 200 has an area of 198 cubic inches. The 300 is bigger with an area of 375 cubic inches. As you can see, the 300 is almost twice as big. I'm not sure what bladder will work or is compatible with each but I would think Weight and Area should be factors. I would suggest a smaller 1 liter bladder for the 200 and maybe a 1.5 liter for the 300 would work fine. Remember, if your walking great distances, the weight of the bladder should be a Prime concern.