"The OKI's I believe are the same engine as the more expensive Xerox's" >>> Absolutely not!!! The only time when they came close is when Xerox sold a Phaser 7400, they took a OKI C9800 engine (a 600x600 dpi version) made their own UI and Image Process Board (AKA "Formatter"). Beyond that, newer machines from xerox all have LED arrays instead of common Laser Imaging Unit, for example Phaser 7500 and Phaser 7800 and all color copiers based on these engines so if anything Xerox is following OKI's lead in this technology.
As far as some pros and cons:
Xerox kicks a$$ as far as print quality where OKI is more durable and handle thicker media because of a straight paper path.
Xerox printer will put paper trough 2 of 90 degree turns for anything to be printed.
OKIs have shorter drum life because image transfers from drums directly on to media and this makes it more prone to paper dust/lint contamination.