the only difference will be the muscles used =)! u'll tone up a bit more in u'r stomach arms and shoulders as well as legs running , as apposed to cycling where it'll concentrate more on your legs but 300cals is 300 cals as long as u'r dippin into those fat resources thats the main thing so choose the exercise u enjoy the most or do both and alternate to relieve boredom,
also the time and effort it takes to burn your 300cal can differ, but that very much depends on your heart rate, your intensity level of training and where your strength lies (u may find 1 easier to do than the other)
=)