Answer:
Internal validity represents the causal relationship between the subject variables in a study, whereas, external validity, containing procedural variables, represents the generalizability of the study and how well it generalize to a particular population.
Explanation: Internal validity is all about control. External validity is all about naturalness and generalizability. My preference is dependent on the purpose of the study. I a study needs to have a large amount of applicability, I favor external validity. If a study needs to be replicated or evaluated to determine if there are any other variables that may be producing the results, I prefer internal validity. In closing, it is difficult to pick just one, because so much is dependent upon context.