I've only been to Reading a few times, but I've managed to get stuck in long queues of traffic every time! Using the Park-&-Ride system would probably be a better option than trying to park in the town centre:
http://www.reading-tr...Information/ParknRide
Access to Watford is easier. Simply follow the signs from Jn 19 of the M25. Car parking is well sign-posted and surprisingly cheap:
http://www.watford.go.../car-parks---council/
Shopping in Watford though isn't a particularly great experience. The Harlequin Centre is where you'll find most of the main stores
http://www.theharlequ...%20Guide%20SS2010.pdf
with a few other shops around the town centre.
I've not done much shopping in Reading, so I can't tell you much about it. However the main shopping centre is The Oracle:
http://www.theoracle.com/website/
Chris