Users commonly look at the sensor resolution of the camera when evaluating gel documentation imaging systems/Western blot imagers. However, it is just not all about resolution. Dynamic range, sensitivity and camera lens all affect how good a camera is. The higher the resolution of the camera is, will enable you to see fine detail in your images making it easier to distinguish close bands on your gel or blot.
A wide dynamic range allows you to see both your strong and weak bands on the same blot. If your strong bands saturate, you will be unable to quantify your data, therefore, make sure you pick an imaging system with a good dynamic range.
When looking at lenses you need to consider the F stop. Smaller F stops are especially important for chemiluminescence as smaller F stops let more light into the sensor and can drastically reduce exposure times.