Although if you get mossed up and stall one motor, it is still drawing full voltage but with zero CEMF to resist current flow. Could burn out (or at least damage) a motor. In series, the still-running motor prevents the stalled motor from seeing max current. On the other hand, with brushless motors like Steve's using, the ESCs will either trip on overcurrent as they try to turn the stalled motor, or they'll fry themselves, sparing the motor haha. Thus far, my brushless ESCs seem to be quite adept at protecting themselves, so I'd say go parallel at the inputs to the ESCs from the battery, as that'll give each ESC more voltage to work with, which is a good thing. But it will have no direct effect on the motors' performance.