Should I marinade brisket for smoking overnight, or only do a dry rub?
Marination is a surface treatment.  So, you are not getting any added benefit by marinating over night.  The only thing that generally penetrates is salt, which might happen, depending on the quantity of soy sauce you are using. So...probably doesn't matter too much.
If you want to &quot;get into smoking brisket&quot;, I would recommend a traditional approach.  Brisket is challenging enough as it is.  Master the traditional approach, then deviate if you want to mix things up.