Well both are for adding flavor to the outsides of the meat (yes there is a tenderizing component in marinating also). Depending on what flavor profiles you are going for you can use one or both to achieve what you want. Typically with chicken pieces I tend to marinate and then baste with a finishing sauce at different times.
As for pushing the flavor into the deeper parts of the meat that would be a brine or injection.
With all of this, the best "method" is determining what finished product you like to eat, and then determining which method helps you achieve that. And that is one of my favorite things about cooking, trial and error until you get the bite you are looking for.