they are in many cases interchangeable but there is a slight difference:

insurgence is an abstract noun describing a state of budding revolution
insurgency is an abstract noun describing a political or military movement as a result of a budding revolution

When in doubt, consult a dictionary:

http://www.merriam-webster.com/dictionary/ins...
Main Entry: in·sur·gen·cy
Pronunciation: \-jən(t)-sē\
Function: noun
Inflected Form(s): plural in·sur·gen·cies
Date: 1803
1 : the quality or state of being insurgent; specifically : a condition of revolt against a government that is less than an organized revolution and that is not recognized as belligerency
2 : insurgence


http://www.merriam-webster.com/dictionary/ins...
Main Entry: in·sur·gence
Pronunciation: \in-ˈsər-jən(t)s\
Function: noun
Date: 1847
: an act or the action of being insurgent : insurrection