Even when the per unit price is equivalent, the quantity purchased is important. In the case of perishables, I'd probably err on the side of a small quantity at a reduced price. I can't drink a whole gallon of milk before it goes bad, so what good would 50% more be? In the case of nearly every other consumable, I'd prefer 50% more because it means a longer period before I have to restock, if I'm careful.
Surely if the price works out the same per unit, it's always better to have the discount than the quantity increase. If you prefer to get more so you don't need to restock then great, you can buy more of it at the same time and the cheaper price means it's just like they offered you a quantity increase instead. On the other hand, if they offer you a quantity increase and you don't want that much... doesn't help you.
For example the shell garage across the road from my office nearly always has an offer of "2 bottles of coke for £x" - if I'm buying a drink there, it's because I'm thirsty at this point in time and don't have anything in my office fridge, and quite likely I'm about to be walking somewhere, buying two doesn't help me, even if there's a good chance I'll want a second one the next day.