I don't know, the I-Rex didn't really employ "smart" tactics when fighting the T-Rex, it pretty much just used it's greater bulk to overpower. It's definitely smart/cunning though and if this isn't a face-off but rather a stalk your prey, it has the advantage there.
So we have a custom-made dino based on a T-Rex vs another based on a T-Rex that evolution has refined over 65million years.
That's a fair enough assessment but when things are overall seemingly equal I'll generally side with the one that has any advantages over the other.
If they are the same size I do not think the V.rex has any advantages in that case. While the I.Rex has similar stats, longer reach/claws on it's hands, and has better problem solving skills.
Although, in checking their physical stats, I think Ill take that back, as the V Rex is bigger than I thought it was. Still think the longer arms, smarter brain, and additional abilities of the I Rex make it quite close though.
the Indominus has forearms that can actually be of use and she's smarter as well, as seen in combat with the Ankylosaurus, the ACU and the Raptor skirmish before the T Rex was unleashed, I Rex beats V Rex handily id say