for your motor direct soldering with the shortest leads you can get away with is best. along with direct soldering to your battery the two together would maybe decrease voltage loss by like .10 volts(thats a guess it could be more than that) but any gain in voltage to your esc and then from there that you lose as little volts as possible is good. but ultimately its all personal preference and how easy you want your time at the track to go. if your turning your motor every other run plugs make it easier and for you batts then anyone could help you if end up in back to back races they could plug your pack in the second car and put it on the track for you and you wouldn't have to worry about a mistake. even when your esc is off it drains power.
plus you don't have to cary your iron to the track either. its up to you though.