Like other people said, the MaxxAir is better in the rain. Also, it's better in the wind, which other people didn't mention. The Fantastic fan has a flap that opens up, and it has to be closed in high winds or it gets pulled around by the wind and has a risk of breaking.
I have a Fantastic Fan. It closes itself in the rain (mine has the rain sensor), but I have to close it manually when the wind starts being a problem. BTW, I'm talking Wyoming/Colorado winds up in the 40+ MPH range. If you camp somewhere where "high wind" is 10 MPH, disregard the comments about wind.