I would choose Labidochromis caeruleus over Tropheus duboisi. The Labs are much easier to keep, less aggressive fish, and they fit in well with the other two species - all of them being mbuna from Lake Malawi.
Tropheus come from Lake Tanganyika, and are a lot more sensitive to water conditions, food type and quality, and so on. Before you keep Tropheus, you really want to study up on them, and dedicate an entire tank to them to house a larger group.
Personally, in a 55G I would go with two species of mbuna. Three at the most. In my humble opinion four is pushing it in a tank that size. I find that trying to cram too many species into one tank is a common beginners mistake. People often assume that more species will make a tank more enjoyable to watch, but fewer species means you can have more individuals of each. With fish like mbuna larger groups mean you will observe more of their natural behaviors, which in my view makes for a more interesting tank. YMMV.