Fuel cell fender will generally accomodate more fuel for the longer race tracks.
Rumor has it that someone ran 7-1/2 laps at Mosport last year before running out of fuel.
Fuel cells for onboard use can be purchased by volume with the "sponge" in place.
The normal location for these is in the nose cone on the chair.
Each of these have thier advantages based on where you want to place some extra weight, i.e. nose for loading the front end or fender for the extra counter balance.
My experiance is only with the electric pump, you have a positive flow to the carbs and anything extra is returned just like in your car. The only problem is when they quit, they quit. Fortunately they are not to expensive.