Usually the concierge cabins are the more central ones, but other than that they have the same size and layout as "regular" balcony cabins. The only real perks of concierge are fresh fruit baskets, and canapes (if you care for them) delivered every day about 4 p.m. I've heard some people say that room service is faster to the concierge cabins. They also earn you an extra point toward Celebrity's frequent-passenger program, but since you're already diamond on RCCL you'll most likely get all the perks of that, anyhow. For these limited benefits you pay an extra $2 per person per day gratuity.