PS2 less powerful than Xbox, PSone less powerful than N64, both sell more. PS3 more powerful than 360 & Wii, sells less... less powerful seems to mean more sales
PS One released first and it received a lot of third party support that wasn't happy with the fact that N64 didn't have a cd rom drive ( ie. no "Final Fantasy" titles on N64). Due to the success of Playstation One, PS2 came out of the gate with an untouchable third Party lineup. This gen, Sony's finished (or will finish) second due to having the best exclusives that were on the cutting edge of technology.
If Microsoft's NextBox is by far the most powerful console next gen, it will be the only console to get the type of third party software support that both PS3 and 360 did this gen.
Sony also lacks the advantages of the Wii U as well ie. being first out of the gate, destined to be the cheapest, and having popular series that don't depend on their technological prowess to attract customers. Sony customers want advanced Blu-Ray tech and games that wowed them like "Uncharted", "The Last of Us", Heavy Rain / Beyond Two Souls" with unparralelled experiences the first time out of the gate if PS4 can't do that its customers will follow the third parties to Nextbox.