Speedmaster... breakable? Well it's no G-Shock but there's people still wearing the ones they bought in the sixties, so they seem to hold up very well. Their design has never changed much, but the special editions are sometimes a little unnecessary. They are genuinely used in Space environments and they can take a beating.
The seamaster 300M you're referring to will be the ceramic bezel type? Ceramic can crack but it's rare this happens. If it did though, you're looking at several hundred pounds for bezel insert replacement. The Seamaster is a wonderful watch but some people dislike the skeleton hands, others love them.
They're both icons in their own right and both very good for everyday wear, but there are better choices if what you need is absolute rough and tumble ability. Perhaps a Sinn?
Another Omega to consider would be a sword hand Seamaster such as a 2254 or 2264. You won't find one new most likely but they're an incredible used buy.