I own both types of bikes from bikes direct. fantom cross and a sturgis fatbike. Both great bikes for the price, I have no issues with them and their business model (some people do and some LBSs will not like you). For everything besides the trails (with rocks and roots) the fantom cross bike would be perfect. If your for sure going on the rocks and roots, get the fat bike.You might want to up your budget to say $800 as you can get some nicer upgrades at that price such as hydro disc brakes.

I would also recommend going to their sister site, bikeisland.com, its their scratch & dent site. My fatbike is from there and it was $150 off because of some scratches on the chain stays. I have yet to find those scratches but added plenty of my own. downside with that site is you will have to dig to find what you want (sometimes they sell 3/4 of a bike).