In reply to Neal.

Neither one is necessarily better than the other, as each has its own list of pros and cons. Hesalite is the crystal material used on the official NASA-qualified model; however, this was primarily because a sapphire model did not exist back in the 1960s when the Omega Speedmaster was first being qualified for manned space flight. A plexiglass-style crystal was initially requested due to its ability to stay together should it receive a strong impact and crack and break, but a sapphire crystal would likely also be able to survive the same tests that were conducted by NASA back in the mid-1960s. Additionally, it is also worth noting that in the years since the Speedmaster first gained its flight-qualified status, there have been many watches with sapphire crystals worn in space and qualified for manned spaceflight, although the classic Hesalite model with a manually-wound movement remains the only one qualified for EVA.
That being said, a solid stainless steel case-back would certainly be preferred over a sapphire exhibition model for actual manned space missions; however, this does not allow for a view of the beautifully decorated movement within, which is something that is certainly an added value for many colletors.