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They’ve actually got the pictures the wrong way round there.
Picture on the first one is hesalite (acrylic) crystal, second one is sapphire. Sapphire one also has a sapphire caseback, hence they do a bit more to the movement in terms of presentation, which also bumps the price up.
You can tell the difference in pics because the sapphire crystal has a white “halo” around the edge that the acrylic crystal does not.
This https://www.beaverbrooks.co.uk/0103836/OMEGA-Speedmaster-Moonwatch-Professional-Chronograph-Mens-Watch/p
vs
https://www.beaverbrooks.co.uk/0010263/OMEGA-Speedmaster-Moonwatch-Professional-Chronograph-Mens-Watch/p
Spendier one seems to have an open caseback.