You can find details of the JAA PPL in the LASORS publication which is available on the CAA website
www.caa.co.uk/docs/33/lasors2005.pdf
The NPPL details are available on
www.nppl.uk.com
Breifly, the main advantage of an NPPL is fewer hours to the licence and less stringent medical requirements. The downside, depending on your intentions, is that you are limited to UK airspace during the day - i.e. no licence 'add-ons' are possible, except for upgrade to flying 'complex' aircraft weighing less than 2000kg.
I would suggest you go and talk to your local flying school. I guarantee they will have had a lot of queries like this and will be able to give you some advice. I wouldn't worry about them being biased either - in my experience they'll be glad to see any new customer, whether for NPPL or JAR!
The private flying forum may also help if you do a search on there for details....
Best of luck.