I would say a general rule would be that GOS will put fat on more quickly than OSB, who are generally slower to finish. Tamworths will look lean but will be fatty.
I would also say with your first weaners you will make mistakes and the next time round you will get a better finished pig. You are aiming for a finished pig not a fat one.
Generally people start with two but there are merits in having three. You can send one before the other two and so you get some idea about booking the abattoir, what cuts of meat you want, etc.
If I'm asked what are the best pigs I would say that you will find what breed does well for you on your holding, that you enjoy keeping and eating. That's why we don't all keep the same breed.