IMO who is better, should read who is most valuable.
On offense they're similar, both are scoring point guards, meaning shooting guards masquerading as PG's. (And in todays NBA that's fine.) Both pass the ball as an after thought, both handle the ball very well. Both are "go to" guys in the 4th quarter, IT with a slight edge. We know what a healthy IT is capable of, Irving not so much considering LeBron mostly runs the team.
On defense, sorry IT fans the little fella needs continual help on the defensive end, he causes the necessity for players to switch from their own defensive assignment to help IT. It's a sometimes confusing, energy drain that manifests itself in the 4th quarter. He is a pathway for the opposition to find a basket. Unfortunately this limitation shows it's head mostly in the playoffs, and can never change.
While Irving is considered a poor defender he has normal size, he has no physical limitation, he can improve his defensive game with work, and study, and coaching. He can some day be a decent defender.
This must all be taken into consideration, when you consider value.
Irving is younger and signed for two years, and thought to be healthy. IT is looking for "the Brinks Truck" shortly, and is showing wear and tear physically.
My heart say IT, my head say Irving.