Okay, I'll have a go at this as the question has gone unanswered for a while. My understanding, incorrect as it may be, is that many boats affiliated with the larger "name brands" are franchises, and there can be great variability in size, age and amenities within a given fleet and between operators and locations. I think that you would therefore be best served by thinking about what sort of amenities you may be most interested in, and what sort of boat features you desire. You can get massages on trimarans, soak in hot tubs on catamarans, sip good wine on monohulls, and lots of variation in between. And for my two cents worth, I've been on some very well maintained older vessels like the Cuan Law and Caribbean Explorer II in which vessel age was not a factor for me at all. If it's an older vessel still in service in the competitive liveaboard industry, there may be something solid about its design or layout that keeps it in business. Have fun making your choice.