They're both big and strong, Fury is probably weaker than Joshua overall though.
What do you mean by weak, not having a body building kind of body 
, i bet Tyson Fury will win against Joshua simply because of his boxing skills and the confidence of an undefeated fighter and he always rises to the occasion.However, the size and reach difference should be enough to compensate for that, plus Fury has a chin sculpted out of granite.
There is no doubt about his chin and he took the hardest punch of Wilder where no one was able to do and went into aggression mode after that.I think if AJ and Fury went toe to toe, Fury would probably know him out. After all, AJ goes KO'd savagely by Ruiz, there's no way that midget would do the same to Fury.
There is no point in having a fight between Ruiz and Fury but Joshua is the next big fight for Fury and once he unifies all the belt i am sure he will retire from boxing as he achieved everything he started and will reach a legend status that no one achieved.