If I read the specs correctly, the Pit Viper fan is a 10 CFM fan and the Pit Bull is a 25 CFM fan. The Pit Viper draws about 0.25 A and the Pit Bull draws 0.5A
This larger current draw will drop your battery power much faster than with the Pit Viper.
IMHO, you don't need hundreds of high power air blasts to control temp when a smooth, steady flow will do the same thing and not necessitate your fan starting and stopping 20 times per minute. I also think that hundreds of high powered air blasts will blow a lot more ash into your cook chamber.
Every air blast is a rise in temp and every turn off is a fall in temp. The perfect scenario would be to get your fan to run continuously at one speed that keeps the temperature steady on the set point and not continuously rising and falling above and below it.
This is far easier to achieve with a smaller CFM fan. Unless you are running a huge cabinet, the Pit Viper should be more than enough.
You can also add a DC motor speed controller in line with your motor and reduce the speed until the fan is on nearly all the time with no temperature dips. A speed controller would also make your battery last longer assuming you were running the fan at a lower speed.