The Centaur is a simpler charger than the Phoenix. It still is a three stage electronic charger but it doesn’t have the same bells and whistles as the Phoenix. For example it doesn’t offer temperature compensation with an external temp sensor attached to the battery, it just has an internal temperature sensor. Also the Centaur just does a three stage charge but the Phoenix does a four stage charge which is more sophisticated. Programming options with the Phoenix are more extensive and it allows you to do an equalization charge from time to time. It can be used with a remote control panel and it has a “once a week” mode for winter storage. The bottom line is that both are excellent chargers, both are multi-voltage capable so they can be used anywhere in the world, but for a bit more money the Phoenix offers you a few extra features.
The typical ratio of battery size to charger size is that the battery can accept 25% in amps of its amp hour capacity. So a 100 amp hour battery can accept 25 amps charge. AGM batteries can accept a charge faster than that but I don’t have an exact number to put on it. So a boat with 200 Amp hours of batteries could make use of a charger of 50 Amps or more. However the question is do you really need that much charging. Unless you have a generator on board once you start charging you are normally going to be tied up to the dock for the night and have all the time in the world to charge the batteries
One other difference between the two chargers is that the Centaur has three equal outputs but the Phoenix has two full power outputs and a trickle charge output for a starting battery.