At work we have a Canon C5045i printer, for which Canon provides three PPD files: ps, pcl, and pxl.
Is there any one that is preferable over the others? Is there any difference between the three?
PostScript and PCL are the most commenly used Page Description Languages, that means the low level language that printers understand. PCL comes in different versions (see http://en.wikipedia.org/wiki/Printer_Command_Language), PXL being PCL-XL or PCL6 (Enhanced) being the latest and greatest version. Most printers only support either PostScript or PCL (a specific or several versions of it). According to it's specifications it looks like your printer model seems to supports both PostScript and PCL6 and 5c.
Generally you should use the PostScript driver if your printer supports it since this will give maximum output quality. The PXL or pcl6 driver can be used as alternative to PostScript if the printer has problems in PostScript mode, like interpreter bugs or low memory. The PCL(5c) driver will probably give you no extra benefit (perhaps the PXL is only b/w and PCL is for color, depends on the info inside the ppds and how well your printer supports PCL6).
The "best" driver really depends upon what you expect of it: speed or maximum output quality and also depends on the input that is used (b/w or color and text/line art or photo). It could be that the PXL is the fasted for just text in b/w while the PostScript gives beter output quality for photo in color, just do a small test to find out or use them both for different purposes.