BBQ - If you're confident in your ability to complete within the 6 hour cutoff, then there's no reason why you shouldn't be in the Race. You'll be further back in the field than you would in the Challenge but somebody finishing in 4:30 is neither going to be winning nor coming last in either.
Both events are over the same route, cost the same and 'qualify' you for the same finishers' rewards including entry into the same spot prize draw. The only difference is that those in the race get to start two hours later - time for a lie in or leisurely breakfast or both.
Don't know if that helps?