Personally I say neither. I do marinate briskets sometimes, <whispering> but its when I cook them in the oven and not on the smoker. I find that an oven baked brisket needs something and my maninade furnishes it. I don't know I have ever injected, I see some folks doing it to add flavor depth or increase their taste profile but a good smoked brisket is good enough for me. If I was cooking against 10 other people I might use one to help with the edge, but now for family and friends.
Brisket (Check List)
The meat, I always use a fuller packer, but it is more about your cooker size?
Clean, dry, trim fat or not to trim, that is the question.
If leaving the fat, fat size up (free biasing) or down (so it doesn’t wash off the rub)?
The rub, store bought or homemade? The glue for the rub to stick to the meat or not? How soon to apply the rub before the smoke? Injection? Marinade?
The smoke! Briskets can take even the hardest smokes, mesquite and hickory work but don't go over board. Foil or no foil?
Rest time, slicing or pulling?
Suggesting; Cooking temp 225 degrees, duration approximately 1.5 to 2 hours/#, Cook to approx. 175 degrees, foil to 190 degrees, bag and tag 1 or 2 hours use a toothpick to test doneness (approx. 203 1/2 degrees). Pull and allow to cool/rest at least 45 mins. Before attempting to carve or it will crumble. Still good just dang messy.
Everything else is personal preference and some preferences do affect cooking times.
While you are at it, take notes, yes, write it down. What you do, and what it tasted like, and what you want to change next time. Or memories are not a good as we think.
Hope that helps ya, relax, and enjoy it. A Guru friend has a single word that is the most important ingreduient is smoking, "Patience".