Parallel gives you same volts, double amps. (4.2 volts, double battery rated amps.) Series gives you double volts, same amps. (8.4 volts, same rated battery amps.)
Differences are when you build @ 4.2 volts you can build down to say .125 ohms with out any problems. If you are using a mosfet though it's recommended to stay above .5 ohms because mosfets are not rated to go much under that. This would be basically the same thing as running a single cell mechanical mod as far as performance. Just now you have a longer battery life that you are using 2 batteries instead of one. @ 8.4 volts you are increasing your output by double and a .5 ohm build will be blazing hot. You should now be building coils @ .8 ohms and still be just as hot as a .5 ohm build. From what I have noticed the coil will light up faster as well it seems.
If you want to get into the math of it find a ohms law calculator and punch in the numbers. Just google ohms law calculator. All you need is two numbers to get a result. You need your ohms. Say .5 for your ohms and 8.4 for a series volts. .... which would be 16 amps and 145 watts output.
link to the calculator:
http://www.ohmslawcalculator.com/ohms_law_calculator.php