Put simply, the M1 is quartz, very big, but not stupidly heavy due to being titanium. The SA is automatic, huge, and weighs somewhat more being steel.
Despite looking quite similar, they are rather different beasts.
I've got the Avenger Seawolf Chrono, which is effectively a steel M1 and kind of fits between the two. There are many days when I wish I'd found a good M1 as it would have been cheaper and much lighter, but then there are other days when I wish I'd bought an SA, as I think the proportions are slightly better (though it is huge and would make me look like a 6-year-old borrowing my dad's watch)
Prices on these are quite odd; M1 prices should be low due to the age of the things, but tend to hold up, especially with the yellow faces. Some fairly new SAs have changed hands for about £2k, despite the higher ticket price, probably as they are a bit too drug dealer/rapper/6-year-old borrowing dad's watch for some.
Suggest you try both on your wrist and your choice might be a lot easier :-)