My brother has two Lagoon 500s that work commercially. Despite what some may say about the brand they are built to take a knocking and are really good boats. They are probably better looking and sailing than many of the other Lagoon boats so will probably remain popular and possibly hold their value well if looked after. I think you can compare them to an Amel 54 in so far as the price range and ability to carry 4 people around the world on the milk run are similar. So I can understand why two apparently completely different boats may end on the same wish list.
I think when comparing the Lagoon to most monohull production boats the Lagoon would come out on top. However, as you are comparing with an Amel 54 it is a harder decision. Although the Lagoon is up to the task it is built to a price. The Amel is much better built. The Amel has many safety features if you are possibly going to encounter bad conditions.
The Lagoon 500 has advantages: the view from the saloon, the privacy of having cabins in separate hulls, the verandah type cockpit, the great view from the frybridge in calmer areas or fine weather, etc.
Disadvantages include the size on the fully battened mainsail if you have a problem, the engines, like most cats seem to have more problems because they are basically in the aft lockers, the saildrives can be a headache for maintenance etc plus the other cat issues of slamming in head seas, jerky motion in a seaway, and berthage.
Either boat would be great to go around the world on but if you chartered for a couple of days It would be very likely the cat would win. On the Amel you would feel more pride of ownership I think, and be much more confident, say, sailing south of Madagascar which is what you have to do if you intend to sail around the world. With the Lagoon you would have more comfortable apartment-on-the-water type amenities, especially at anchor. I personally would easily choose the Amel and I have 2 children, but it depends upon the families priorities. My wife would may prefer the cat until really bad weather in the open ocean, then she would want to fly home from the next port!.
Cheers John ,Maramu, Popeye #91