I own a classic hesalite Speedy, and the IWC Pilot Chrono you're contemplating. To be specific, I own the black dial version of the Pilot Chrono.
What drew me to the IWC Pilot Chrono is the design of the dial. It's well-thought out, simple yet evocative of of a complex aircraft instrument panel. Little details like the concentric ridges in the sub-dials are appealing to me as well. It's super-legible -- it's the only chronograph of mine that I can read (including the totalizer dials) while hiking at night with a headlamp. Though it is a full millimeter larger in diameter than the classic Speedmaster, it doesn't seem to wear appreciably larger. And I like the idea of owning a watch driven by the workhorse Valjoux/ETA 7750, which IWC can tune to chronometre-like precision.
On the downside, the watch is much thicker and heavier than the Speedmaster; I don't have the numbers at hand, but the weight and thickness difference is really noticeable when worn. I tried to wear the IWC on a NATO strap but the strap wasn't rigid enough to keep the watch under control -- just too much mass (maybe due to the soft iron inner-case?). The black leather strap that comes with the IWC is thick and unusually stiff, and necessary to manage this watch. So that's how I wear it -- with the original strap. Very happily, I should add
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