People have spoken favorably of the H3 (SubGravity and SeaBear are the same thing). The xDeep/Genz Black has a couple long threads here that are worth checking out.
I have an xDeep BlackBT and it's alright, but certainly nowhere near as nice as any of my Shearwaters.
Screen size is too small. It has the same screen size as the H3 (1.7"). The T1 just has a larger body and the wheel interface. The benefit of the H3 is the smaller form factor, just a little bigger than watch size. The T1 really doesn't get you anything other than a different style of interface, which, despite having yet to play with it, I can't really see that it would be that much better than a 2-button interface. I can't imagine that it would be impressive enough to get a Shearwater user to switch, and will probably appeal to new buyers, or technophiles. I have been known to be wrong before.
The Petrel has a 2.4" screen. Doesn't seem like much, but in fact is much larger. Of course the whole computer is much larger as well.