Both are great and funs shows! The expo is bigger at the Arnold, and there are a lot more people, but 80% of the people are not bodybuilding oriented (aka, cheerleading, ping pong, karate, and 25 other competitions). Still, 100,000+ people go to this expo and events, which makes for the 2nd largest event in Columbus, Ohio, and one of the best. The Arnold rivals the Olympia in terms of prize money, although in recent years, the Olympia has generated some of the highest prize money ever.
The Olympia crowd is more bodybuilding oriented, about 25,000+, but 80% here are for bodybuilding, this making this event a more prestigous event for the bodybuilding crowd. It features the top bodybuilders of the world, and is considered the crowning acheivement of bodybuilding.