Thanks everyone,
As I couldn't decide I've been using both for the last few days for GNAV and have only found one question that's in the Bristol QB and not in Keith Williams CD. It may be there, but that is one negative about KW's CD, it doesn't have a good search function, althoug they are split into subjects areas which does help a little.
I must say though, KW's CD does win when it comes to the explainations and calculations.
Furthermore you pay once for the CD and it's yours to use when you want and cheaper than Bristols.
I guess though with Bristol they are constantly looking at and updating the question structure etc, but you only need then for a year or even less.
Thanks once again.