GPA vs Undergrad prestige and difficulty
Admissions don't come down to a single number or any single factor. Knowing no more it is impossible to say which of these two is a better candidate. 

While it is true that some universities are considered "better" than others, it is an ephemeral thing. Some marvelous teachers work at some quite "modest" places. Moreover, the universities that have a really great reputation, often do so because they are able to offer more learning support to undergraduates, though that isn't always the case. They can often attract top faculty, of course, for financial reasons, but that doesn't necessarily translate to better undergraduate learning. Certainly not for every student. 

Someone with a low GP from a top university, might have been somewhat disconnected from the learning process and had little contact with those top professors. Someone from a lower ranked place might have that higher GPA because he/she worked closely with the faculty and has a much better grasp overall. This, too isn't necessarily true, but it can happen. 

Another factor, of course, is that not all of the "top" students go to the "top" schools. Money plays too big a part of who gets to go to, say, Harvard. Some people just go to a local State University of modest reputation due to family or other reasons. But they can get an excellent education there, and have done. 

And don't assume that John had an easier path or that his university had a "significantly easier curriculum". There is no basis for saying that. Some graduates of highly ranked places are also quite weak in skills. 

However, I suspect that there is a general feeling among academics that a student who has done well locally (high GPA) has taken advantage of the offerings and is well prepared, while one who has not done so well in the local environment may not be as well qualified to continue. But that "general feeling" needs additional evidence, so isn't considered definitive. 

Admissions is a game of balancing. Many factors are looked at, grade point and college reputation among them. But those alone won't tell a useful story about a candidate.