From what I have seen and experienced there seems to be a fairly even split in Stockton between Boro and Blunderland fans. Maybe Boro edge it. Middlesbrough and Redcar, Guisborough, East Cleveland are very strong Boro bases. Boros Heartland.
Most of the way down the coast to Whitby its Boro, with some Leeds from Staithes/Hinderwell and then from Whitby its a large Leeds following. They have regular fan coaches from Whitby to watch Leeds I believe.
A very unscientific research finding I admit.
UTB.