The biggest noticeable difference I have seen between the current Kryptonite and OnGuard U locks is in the fit of the shackle to the crossbar. Fit seems to be much tighter on the Kryptonite lock with almost zero free play when the parts are locked together. Key operation also feels smoother on the Kryptonite. Overall the Kryptonite just seems to be a higher build quality unit.

This is comparing a medium size narrow Pitbull with a NY Fahgettaboudit U lock side by side. Overall both are similar size locks though the NY is a lot heavier due to having an 18mm diameter shackle versus the Pitbull's 13mm diameter.

No real idea of the differences in steel quality on the two locks though the larger shackle on the NY lock should slow down power tool attacks.