Deciding iOS version: pros and cons

Deciding iOS version: pros and cons

I could not come up with a decision about it recently, and decided to give
it another try.
Assuming, one may be starting to develop an application say in a month,
can you please help me come up with pros and cons of:
targeting the application only for iOS 6 (we could include iOS 5 too - I
think there are no big differences)
targeting application only for iOS 7
targeting application for both iOS6 and iOS7
ps. as I said when I mention iOS6 we can assume iOS5 should be supported too.