The toomey is usually considered in a different class than the Fatty.
The Fatty is a general torque pipe, it makes more power than stock over about the same rpm range as Stock. You gain more power over stock above 4k or 5k rpm IIRC, with a slight loss below that point.
The B1 is more of a power pipe, and tends to make the most power above 6k rpm on up to peak about 500 or so rpm above stock, then pull nicely through over-rev.
In a flat out race across a flat up a hill etc. the Toomey will win nearly every time. Through slower technical twisty trails particularly with limited traction, I would expect the Fatty to be easier to handle and able to make the same or better pace.
Most of us like grin inducing power, few of us professionally compete. It really comes down to how you ride and what you want.
If you like the way the blaster runs and just want some more grunt and a more noticable hit, the Fatty is your ticket.
If you are more high strung and want a stronger more usable mid range, with a heavy hit and more overrev, and rarely find yourself lugging along the the Toomey is your pipe.
Every pipe has a personality, the better a pipe matches the personality of your motor the happier it will be , the better a pipe matches your personality the happier you will be.
Best thing to do, is read up on the pipes your interested in, and find people with those pipes see how they like it, where it works and were it's lacking. If you have engine mods porting, head work etc. try to find people with similar mods.
If you are not sure, or just on a tight budget you can always watch for deals on C-list, Ebay, etc, buy used and if you decide you want to go a different route you can often pick up a different pipe for about the same money that you can sell the first pipe foe.
It takes time and patience to go this route. The fast track is to buy a couple pipes you are interested in try them, keep your favorite, sell the others, or save them for a friend or relative.