I dont think theres much between the 2 to be honest, i think i read somewhere on this discussion that one is better formulated - probably the Shell fuel - but cant remember, but both should end up giving around the same bang for buck as it were, altho Tesco is usually a few pennies cheaper by comparison round my way.
I tend to use Tesco tho as its a bit cheaper and the car seems fine with it, plus the double clubcard points are always useful too