“Best” usually boils down to personal preference due to features and experience. But, when you have neither, you can only go by the opinions of other people and look for those who are the most neutral in their comparisons.
Personally, I like the MaxxAir Fan better simply for the fact that it has a rain guard that allows you to use it while it’s raining. The Fantastic Fan automatically closes when it rains. Air flow is a major factor for me and I like the idea of having a rooftop fan to blow the hot air up and out instead of just swirling it around from side to side.
As for having one or two, it depends on what you want out of it. From my understanding, the reason many people have two is to have one over the bed and one over the cooking area. Even as fussy as I am about air flow, as long as you have side windows that can open, and another fan inside the van, I don’t see the ‘need’ for having two. I’m nervous enough about cutting one hole in a van roof! But, if you don’t have windows in your van, or windows that don’t open, then I’m in the camp of getting two of them.