I, personally, wouldn't pay the extra money for a "preferred" location at the CBR. During our first stay (10 days in June, 2006), we were assigned to Jamaica Building 41. It was a great location. About a three minute walk to the bus stop, a minute walk to the quiet pool and beach and about a five minute walk to Old Port Royale. Our room faced a beautiful lush courtyard. I loved everything about the location.
This past stay (10 days in June, 2007), we were given an upgrade to Martinique. The walk from our building was about the same as if we were in Jamaica. Our room faced a very pretty courtyard with bright umbrella tables.
The bus system runs from two different ways. If the bus arrives via the entrance near the Custom House, then the Martinique bus stop was the first stop to pick up and let off. If the bus arrives from what I called the "side entrance", then the Jamaica bus stop was the first stop to pick up and let off.
Unless you really want to pay the extra money for a preferred location, I would suggest a stay in Jamaica. We really liked the view of the courtyards too.
Lisa