The GTBR 2007 will be my 6th Great Ride with BV (and not my last!!)
The second part of your question is easy. It is a cycling holiday, not a race. You don't want to be uncomforable for 9 days!! In my opinion, definately mountain bike, both for the comfort and gearing. You won't be left behind. I would recommend road tyres though, but not a requirement.
My first 3 rides were riden on my mountain bike, and the next 2 on my hybrid.
The ride format is very similar to the Vic rides, so if you search the current forum for the GVBR, you will find great advice on what to take, what to expect etc.