Which is faster, Serial or DigitalWrite?
Using digitalWrite() is the slowest by far.

Using PWM could be the fastest.

Using Serial could be the fastest.

PWM:

You have to filter the PWM output to be a smooth analogue signal for it to be of any use. The filtering adds an inherent sluggishness, or response time, to the system - the harder the filtering the slower the reaction to changes in the PWM duty cycle. Adjusting the filter component's values will give an optimum point between smoothness and response time. Bear in mind that the motors may respond erratically if there is insufficient damping of the signal. This will probably give the Sabertooth a hard time, too. The datasheet recommends are filter response time of greater than 1ms.

Serial:

If you are considering the 2x 60Amp version (thanks for adding plenty of information in the question, by the way) then the fastest baud rate is 384kbps - or 48k commands per second. The Arduino will of course not be churning data continuously because it'll be busy running the rest of the program, but using serial it is theoretically possible to send commands faster than the motor controller's output PWM.

So which is faster?

No idea. Only you can determine this, and I doubt you'll be able to determine the answer without wasting your time on some experiments.

The motors will have an inherent mechanical dampening thus creating an overall bottleneck in the whole system response time. I can almost guarantee that both PWM and Serial methods will be much faster than the response time of the motors themselves.