Neil, a good question!
They are very different events.... LBS at Excel is a big, glossy and very well presented event. The venue is huge, you can't really get around it in one day unless your really keen! It's crowded, quite expensive and of limited value, unless you have a big budget that you need to plan to spend. However, the long distance traveller does have one big advantage over those of us who live relatively close.... the best way to visit would be to fly in to London City Airport (the best view of Docklands you'll ever get on the approach!) and take the DLR (or a taxi) to the front door.
RIBex is a much smaller event, and although Hugo and Steve work hard to make this a showcase event for a cottage industry, it's fair to say that it can't match the professional glamour of the bigger London event (although many would say the the Pascoe girls match any of the glamour on show at Excel!) The location on the Isle of Wight a bit like being on the Isle of Arran (although a shorter water crossing), but RIBex is more of a social occasion as it's easy to meet up with people, and the environment more cordial, IMvHO. There are many manufacturer RIB demonstrations available on the water, and last year the BIBOA race teams did high speed demos on the race fleet (Cookee in "Bananashark" and Mel/Andy Wilby in "Cardinal Sin"). You'll also find many RIBnet members with their boats over at Cowes during the show and we have always enjoyed having both old and new friends aboard over the RIBex weekend. You can also fly to a local airport - Southampton. But book accomodation early if you intend to stay on the island, which I would recommend that you do.