Depends if you like dressing up or not Mike
Revival is more about the "glory days" where period dressing is all part of the experience.
Festival of Speed has, at its central theme, a competitive hill climb open to all genres of sports/racing cars/bikes (from a 1920 Bugatti to an 8c roadster to a suped-up lorry!)
Both are quite family oriented, especially the weekend days. It's for real sportscar enthusiasts (the kind willing to stand at the back of a six deep crowd in the pit lanes just to glipse, hear and smell the experience of these cars firing up and driving past!
Secondary point being, as it's getting increasingly popular evey year, it can get busy!! There's loads of activities to keep even the most short attention spanned of people amused.
I personally get more out of the FOS - as it combines all the latest supercar exotica, race track and pit lane fun. You'll never, EVER get the chance to see, let alone touch most of these cars in tour every day life.
Check out the site - put the 2006 DVD on your Santa list!!
I'd also be up for an AO outing!