Most efficient h bridge to run two DC motors in robotics application
To maximize torque you will want to source 6V @as close to the max current as you are comfortable with. I like the TLE 5205 chip its 7pin hbridge that can handle up to 5 amps digikey http://www.digikey.com/product-detail/en/TLE5205-2G/TLE5205-2GINTR-ND/1283075 has a couple different pin layouts but this chip can handle sources from 5.3-40V and is a hog.  This will be capable of maxing the output for the motor.  You can control it any number of ways with PWM or just forward or reverse bias currents.