The display along with the rest of the user interface--the buttons, the prompts, the menu structure, etc. And the user-replaceable AA battery. It's just so darn easy to use in all respects. Zero aggravation.
I do not agree on the buttons. I think the H3 buttons are not just "as good", but actually easier. Just as one example. I bet the OSTC3 buttons are easier, too. The buttons were one thing I did not like about my Petrel. Especially with gloves on, I often found myself trying to press a button on my Petrel and failing because I was a little off to the side of the button and couldn't actually feel the little raised lip around the button to tell where it really was.
I can easily tell by feel exactly where my H3 buttons are. And if you look at a picture of an OSTC3, I bet you would agree that those would also be much easier to identify and use with gloves on, in low visibility.
In fact, after reading the OSTC3 manual a couple of days ago, I think it is an overall more desirable computer than a Perdix. Smaller in every dimension, including much less thick. Same size screen (technically, 0.2" smaller diagonal). Same resolution screen. Also uses any kind of AA battery. Open source OS. Buttons are easier to find/use. And it's only USD$700. It's a dang shame they don't sell them directly into the U.S.. But, I have been seriously considering ordering one and having it shipped to a buddy in England or Canada and then letting them ship it on over to me.
And, obviously I think the H3 is more desirable than a SW as I had both at the same time and sold the Petrel 2.
That said, a buddy told me yesterday that his "guy" at his local shop told him that Shearwater had contacted that shop to collect market data from them on the possibility of coming out with AI. If that happens and ScubaPro doesn't give me some option for AI on my H3, I may end up as a Shearwater owner again after all.