I have the stock , Fatty, and Gnarly for my '98 300 EXC. That being said, I use the Fatty with a pipe guard.
The Gnarly will give you a lot more noticable low-end, too much low-end in my opinion for a 300, it comes on too strong and too soon for the snotty nasty/muddy/rocky conditions. If you ride nothing but tight sand I would possible reccomend it. The Gnarly is a good choice for a 250 for some one who wants more low-end.
The Fatty gives a more smoother transition of power thru-out the powerband, even more so than the stock pipe.
BP