Potions vs. enchanted items is, as some already said, a decision between versatility and security. Enchanted items always work. And depending on how good you are at making them, they will work a lot better than your average evocation for about the same cost. A potion, even if you have it prepared explicitly, will be gone after the deed, and there is nothing you can do about it. If you don't have it prepared, you might not get to use it at all.
Now for some things, potions make sense. A spell to open a specific door, for example, you won't use that again, and you probably don't have much time to create it anyway. Both factors against the enchanted item but for the potion. And this works very well retroactively.
I won't let somebody roll on a potion or spend a fate point, if it makes sense they'd have prepared something like this. Water breathing potion for an adventure on the sea? Sure. If it's something where I have difficulty believing that the character would have prepared something like this, I let him roll, depending on how improbable I feel his potion is. If I think that there is no chance in hell he would have prepared himself for something this obscure, I'll let him spend a fate point for it. That way it stays within what the character would do, but if he really wants to, he can be lucky and have just what he needs. Maybe he took the potion he wanted by mistake, while reaching for another one.