You'll probably want a Still (Forestry) or Distillation Tower (GregTech) to turn the biomass into biofuel/ethanol first -- this drastically improves both storage efficiency and burn rates.
Once you've done that, it depends on how large of a tank. Steam boilers are the most efficient
/after a long warmup period/, but unless you have tens of millions of heat units, they won't hit that theoretical efficiency. If you don't want to burn 300+ buckets of biofuel at once, a set of combustion engines are the better option, as well as cheaper to build. ((Electric Engines are terribly inefficient, aiming instead for burst power.))
Likewise, storage is going to depend on how much energy. Each bucket of biofuel is worth about 200,000 MJ. For three buckets, you might as well just use a Redstone Energy Cell. You probably want more storage than that, however, which brings you into the EU-verse. Using cobblestone generators and magma crucibles into a thermal generator (or pump and geothermal generator) will get you about a 1 MJ -> 1.5 EU conversion ratio. That brings each bucket of biofuel to 300,000 EU. MFSUs can store 10,000,000 EU, or 33 buckets worth of biofuel, per unit.