Its bound to the older blu-ray to be honest - I had the old steelbook lying around until I sold it a while back. PQ isn't bad but the film is intentionally soft and gauzy looking so its certainly no reference disc. I imagine the Criterion does look better.
If you're a fan I'd definitely await the 4K - that's what I am doing. I'm surprised we haven't had it announced already as we have already had Kubrick films in 4K of course. I'm waiting for Eyes Wide Shut too - now that really does need an update (the Blu-ray really is hardly any different from the DVD and yeah I compared them back in the day!).