I debated long and hard between the Fly vs. AFX. I tried on the AFX-39 at a dealership and it didn't fit me well. My nose rubbed against the nose piece that sits above the chin guard and felt a bit awkward in general (and it's not like I have a giant nose--I don't think so, anyway!)
At another dealership, I tried on the Fly Trekker and was immediately sold. They look like very similar helmets when you check out photos of them online, but the Fly felt much, much better on my head. No more nose rubbing, the padding was more comfortable, and it was just a better fit in general. I also liked the vents and the overall style of the Fly more than the AFX.
Having bought and ridden with the Fly, I must say I'm impressed. For such an inexpensive helmet, this feels every bit as comfortable as my Troy Lee Designs SE3 dirt helmet.
If anyone is debating between the AFX and the Fly, go track them down at a dealer and try them both on. Perhaps the AFX will fit you better, but if your experience is anything like mine, the Fly is well worth the extra $10.