What is best to use, 98 octane or e85?

98 Octane Fuel
Octane is a measure of a fuel's ability to resist a phenomenon known as knocking, which a driver may detect as a pinging or knocking noise coming from the engine.  Knocking is the explosive or uncontrolled combustion of fuel in an engine’s combustion chamber that can very quickly destroy an engine.
The higher the fuel's octane number, the more resistant it will be to knock.
That mean running 98 octane in your car, allows us to give you more power, without putting too much stress on your engine.
All of our dyno tunes are tuned to 98 fuel - unless you’ve requested otherwise. We also suggest you stick to the top brand of fuel (BP/Caltex/United/Mobil/Shell) and try and steer clear of any “no name” petrol stations.

E85 Fuel
E85 can be perfect for the high horsepower or high compression build. Because of E85s increased resistance to detonation, it allows for more power without the cost of expensive race fuels. Even at low power levels, E85 can also be a much safer fuel than normal pump gas. Much like methanol injection, the alcohol present in E85 is great for cooling down your incoming air/fuel charge. This is incredibly important when you are running a supercharged or turbocharged set up. When air is compressed with forced induction, it causes the air to heat up. The cooler the air charge, the denser it will be and therefore the more power you will be able to make. Aside from cooling the incoming air charge, E85 will reduce hotspots in the combustion chamber that can trigger preignition with regular gas.