I guess a Middlesbrough base could be an option. Stockton seems to be the poorer of the two for retail options so I'd regard it as potentially more attractive to start from.
I live in Ludworth, so it's a bus from here to either Durham or Peterlee, then a 2nd change in Stockton (or wherever). Even with a free ride from my Stagecoach staff pass, I've never considered anything other than the car to get to Portrack and probably never would.