Unless you have an opto-isolated speed controller, it is essential that the negative wire of the UBEC is also connected to the negative wire of the speed controller.
This means that if you tap off a single battery, it must be the one that is also connected to the ESC negative. If you tap the other battery, you will short out the battery connected to the ESC negative as you connect everything. This will burn out the wiring at some point.
With an opto-isolated ESC, this is not a problem.
The advantage of having a separate connection for the ESC is that you can test the radio without having the motor armed. The disadvantage is that its one more connector to fail.
I've gone the path of wiring the UBEC to the same connector as the ESC input (i.e. the deans connector). I like the ease/reliability.