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Rather than have two message threads going on for the same question, I deleted your other post in the Carnival section and we'll just use this one for the discussion.
Drydocking means the Monarch will be freshened up with new carpets, bedding, and so forth. No big modifications are going to happen.
The inside cabins on the Monarch are 119 square feet as opposed to the 185 square feet offered by inside cabins on the Sensation. That is something to consider with more than one or two people in a cabin.
Frankly, for a 4-night getaway cruise to the Bahamas I'd look mostly at the price and go with the cheapest fare and not dwell on the ship or cruise line. The Sensation is a nice ship and I think you might find Carnival to be a great choice. I like both Royal Caribbean and Carnival, but the Monarch is getting a little long in the tooth at 17 years of age.