He's a much better prospect than Ed Davis, and I liked Ed Davis as a prospect. One of the biggest difference is their dexterity with the off-hand. Davis is a classic strong-hand dominant lefty that finishes almost everything left. Ivan Rabb has been instinctually doing all sorts of finishes with both hands for many years now, that's why I've long compared him to David Lee..
There's also a big difference with their touch. Davis is well equipped to finish at the rim but once oyu get out of that 5=foot range Rabb's touch becomes vastly superior.
https://www.youtube.com/watch?v=T2XUgFcm-pY
I'd also argue that Davis was playing more of his NBA position in college/HS than Rabb is now, he's asked to play 5 like he was in HS often, in the league he's clearly a 4.
There's Chris Bosh like intangible with Rabb too. He put on the best performance I've ever seen to win a HS state championship 2 years ago. Willed the victory vs an absolutely loaded Mater Dei squad. He brought it to OT and then sealed the W. This is one of the reasons I'm certain he'd fall right in-line in Utah. He's a winner.