As Z80 Refugee says, bipolar transistors are current-driven.
The ratio of (current ito collector / current into base) is called Hfe, and is quoted on the data sheet for the particular device.
However, as the collector-emitter voltage drops towards zero the Hfe also drops, so you need to put more current into the base.
That means you're going to have to switch a large current into the base!
- and with these sorts of current you want the voltage to be as close to zero as possible, because volts times amps is power, and that's just going to heat everything up towards destruction!
- and don't forget that the base current is not at zero volts - base-emitter voltage will be greater than 0.7v with the attendant power
FETs - Field Effect Transistors are voltage driven.
The higher the voltage between gate and source, the smaller the resistance between Drain and source.
A typical device is IRF7787 http://www.farnell.com/datasheets/2042984.pdf
If you put 10V between gate and source, the drain-source resistance will be (typically) 7mΩ. Raise the gate voltage and the resistance drops further.
At 50A and 7mΩ that's about ⅓W
Another thing to remember - it takes time to switch a device on and off.
Switching it on won't be so bad with an inductive load, but while it's switching it off, both the current and voltage will be significant.
Half way, with the volts at 6V and current at 30A, you're dissapating 180W - you don't want that to happen for too long!