Which, in car batteries, is very good if you want to go on a long trip. 
<This is me, hitting my head against the keyboard repeatedly while wondering why I didn’t see that coming.> 
It doesn’t really matter which car you connect to last as long as the last connection is to the frame and not to the battery.
Either battery can explode. Current flowing through either battery causes some of the water in the electrolyte to electrolyze into hydrogen and oxygen.
Also, it’s not just the spark you need to worry about. If you happen to hook up something wrong (or there is a serious electrical fault in the dead car), you can get an excessive amount of current to flow, which can cause the electrolyte to boil. Pressure builds up inside the battery case and KABOOM! This can also happen with either battery.
You don’t need to power the dead battery, but send power to the vehicle. Mostly to the ignition coils, which are grounded at the engine. Going to the dead battery adds no value and puts a connection at the terminal that could spark if the cable is moved. Placing the source (good car) negative right at the battery eliminates any other voltage drops due to other wires, etc.