Well, the rule of thumb is 1.5-2lbs per gallon, if you are going to be using the rock as your source of filtration. So.. in this case, if you just want the rock for show, and are going to be using another source of filtration, then i'd say sure, go for the 15lb box. But if you are looking to use the rock as means of filtration, then go with the 25lb box. How much rock do you currently have in the tank?
So just think, if you're not really on a budget, then to completely setup your means of biological filtration with live rock, then you are going to want 40-45lbs of rock, minimum. Anything less, and you may be taking a chance on whether or not the rock can keep up with the bio-load of your inhabitants. But like i said before, that's all depending on whether or not you are running a separate source of filtration, and not just filtration from the live rocks.