One consideration is the frequency of operation
- a few big ocean liners might do one or 2 trips a year, maybe to Iquitos in Peru
- the Tucano offers cruises almost every week, from 5-7 days
- the Amazon Clipper and Premium offer weekly departures of 3,4 or 6 days
- The Grand Amazon offers weekly departures of 4 or 5 days
- the Jacare-Acu and Awape offer 4-8 day scheduled cruises throughout the year
- the Desafio is supposed to offer weekly 4 day departures
- there are many hammock boats and air-conditioned cabin boats available for private charter for any time and any length of time
Another consideration is what your interests are
- if you just want to get a "taste" of the jungle, then a short 3 or 4 day cruise on the Clipper or Premium might suit you
- if you want luxury and evening entertainment, then the Grand Amazon is probably best
- if you want a good cultural experience, then some of the Jacare-Acu and Awape cruises offer that
- the Tucano and Jacare-Acu and Awape offer the best regular nature experiences, while charters can also offer that. All of those options also offer a more active experience.
- the Tucano offers the best combination of comfort + nature and active experiences (with the option to be less active if you prefer - they separate into 2 groups for excursions - 1 for those who want to be more active and 1 for those who want to take it easy)
- of course if you want a completely customized experience, then a charter is the way to go. You can hike and canoe as much as you want, camp if you want. That can be expensive unless you have 4 or more in your group
- can you handle bunk beds? Some boats like the Clipper, the Desafio and some of the charter boats only offer bunk beds. The Tucano has some cabins with lower beds, as does the Premium and the Grand Amazon
Also be careful when evaluating reviews. Someone who is used to budget properties might post a 5* review, but a person who is used to luxury properties might give it a 2* review. And people might not know what is acceptable or not. I heard about study that said that tourists might rate a restaurant as excellent, since it might be their first experience with that type of cuisine or the first one they visited in an area, while locals might give it low marks because they don't consider it as good as others they have visited in the area.