(1) Weak (global) lexical priority: No amount of lower pleasures can compensate for the total lack of higher pleasures. (Better to be a human dissatisfied than a pig satisfied, and all that.)
(2) Strong (local) lexical priority: Whenever one has a choice between additional higher or lower pleasures, one should always prefer the higher pleasure, no matter the relative quantities on offer. (That is: choose quality over quantity, without exception.)
The weaker version of the view seems much more plausible. (After all, isn't Mill right that few of us would look kindly on the prospect of being turned into a pig, no matter how blissful the pig might feel?) The stronger version is less so: as pointed out in lectures, it seems absurd to think that an extra moment of philosophizing would always outweigh the pleasure of relaxing on a lazy Sunday morning. But then, is there actually anything in Mill to suggest that he held the stronger view?
* Thanks to Helen for this idea.