In a real fight the T-rex will win! This is because the rex'es bite force will easily kill the Spino.
It's also worth noting the following: the Rex has/had a more powerful bite force that could shatter bones, but the film-makers "conveniently" left that part out. Spino's jaws were much more suited to seizing fish or smaller, perhaps even human-sized, prey. Do the math...the Spino would have lost. Also, it is worth mentioning that the Rex in the third film was somewhat smaller than it really would have been...at only thirty-nine feet and the Spino is enlarged substantially to larger than it would normally have been. So...to conclude, if such a battle were to actually take place, the Rex would certainly have won...but the Spino, having size and stronger forelimbs on its side, would be able to put up one heck of a fight.