The difference is in the oil. Let me explain:
Rolex owns all the intellectual property of the mechanisms that they use. They either designed them themselves or involved a consultant, but the end result is that they are in full control of the whole architecture.Rolex also has the freedom to choose how much they spend on manufacturing all the components, and what type of quality and surface treatment they get. And let me tell you that they don’t go cheap on you as a customer. Rolex uses the most expensive and the most efficient oil to lubricate their gears.
Longines, on the other hand, no longer owns any of the intellectual property of the mechanisms that they use. Longines stopped designing mechanisms in the 1980’s to source instead from ETA, the mechanism supplier of the Swatch Group. One of the last mechanisms that Longines designed, the L990, was transferred to Lemania, which was dissolved after being integrated into the Breguet brand. Longines mostly uses off-the shelf mechanisms from ETA, and the only personnalisation consists in having the Longines logo engraved. Those mechanisms come from the same stock that is used to supply other brands of the Swatch Group like Tissot, Certina, Mido, Hamilton and Rado.
Recently though, the Swatch Group started creating small variation of their mechanisms. Each brand could choose to contribute with cash to the development, in exchange for an exclusive on the version for a determined amount of time, until it is made available to other brands.Longines and Omega have also hired ETA to develop special variations that will exclusively be reserved to them.The Swatch Group likes economics of scale, so it always tries to share as many components as possible between a custom mechanism for Tissot and a custom mechanism for Longines. ETA tends to level down, not level up. So when it comes to oiling gears, ETA uses the cheapest oil that offers decent performance, but definitely not the best performance. Longines does not get a say in which oil ETA uses in the movements that are delivered to them.
And that is the difference between Rolex and Longines in a nutshell.Rolex manufactures every element of their watches in house. Longines uses movements sourced elsewhere, typically ETA. Nothing wrong with that but apart from the Longines brand recognition that makes their watches more of a commodity.
From a purely functional perspective there probably isn’t any difference at all but one is definitely more fun to own and wear than the other.