The Key factor in choosing photography and illustration is the “realness” of the design problem in hand. By realness I mean the more you want your design to be real the more you use a photograph, the more you want your design to be abstract and open for discussion the more you want to use illustration.
You can think of these two Photograph and Illustration on the two extreme sides of the scale. and your design problem can be somewhere in between those as shown below.
Your Design (some where in between)
An illustration leaves more to the mind to imagine and give a broader appeal, where as a Photo give a “now” and “here” feeling by showing facts to you, see picture example attached.