The other criteria is how much competition there is. I don't really have time to do much affiliate marketing but I research niches - just for the fun of it mainly. Google keyword then keywords in quotes then google allintitle:"key phrase in quotes". If there were 3 niches you were considering then compare how many searches vs how many competing pages to pick the niche with the most searches vs lowest competition.
Here a couple posts about how I do KW research.
How to do Niche Keyword Research