I guess I'll have to sit in a ZR2 sometime. Last Colorado I sat in I was underwhelmed with cabin volume. Seemed narrower and more cramped than the H3. I couldn't see the gauges b/c the steering wheel was in the way, console too wide for a narrow interior. Just what I observed. On the H3, they pushed the H3 body as wide as their assembly carriers could reasonably take. Just seems a lot wider inside, more legroom too. But can't argue with the ZR2 Pkg. Sure looks great!!
To answer the question about how many parts the two vehicles share: According to the H3 book, what I remember reading is according to a GM designer, the H3 only shared 10% of parts with the Colorado, most of that coming from the drivetrain and dash/firewall. I'll re-read through and verify. That was far less than originally planned.