Carasek's Bag droprate

I'd assume the special box, considering there are less items in it overall. Not only that, there are much fewer common items so the rares would in theory be more frequent.

But this is just speculation.

My speculation is that the rare items have a fixed % drop rate, thus the normal box would be better to buy as it has half the cost.

The special box only has 4 fewer items in it.

That seems to be the bottom line, whether the rates are fixed or not. But I think your idea is more sound. The benefits if it were fixed outweigh the marginal decrease if it were otherwise. But this is assuming the special box rates aren't considerably higher.